Line one 24-cup or two 12-cup capacity mini muffin tins with mini cupcake papers.: The paper liners visually define each tartlet and make removal simple, you should feel the thin paper as you press it into each cavity, and hearing a faint paper creak tells you it is snugly fitted. Use liners that sit flush with the tin so the crust presses evenly, and avoid over handling the paper which can tear. A common slip is forcing liners into warped pans, which makes filling uneven, so use straight sided tins if possible.
To prepare crust, mix together cookie crumbs, sugar, and butter until evenly moistened. Spoon a rounded teaspoonful into each muffin cup. Using the base of a shot glass or an upside down bottle lid, firmly press crumbs into bottom of cups. Freeze until set, at least 30 minutes.: When the crumbs are moistened the texture shifts from powdery to clumpable, you should see glossy specks of melted butter glint among the crumbs and feel a uniform, slightly tacky mixture. Pressing with a shot glass gives a smooth, compact base that will hold the mousse and provide a crisp bite; you will hear a soft scraping sound as pressure compacts the crumbs. If crumbs remain loose, the crust will fall apart when unmolding, so press firmly and evenly. A mistake to avoid is using too much butter , which makes the crust greasy and soft rather than crisp upon chilling.
Pick through raspberries and set aside 24 of the prettiest berries. Puree the rest in a food processor or blender. Strain through a fine mesh sieve to remove seeds; you should have about 1/3 cup of puree.: Selecting ripe raspberries ensures bright flavor and good appearance when topping each tartlet, and when pureeing you will notice a fragrant, fruity aroma that signals readiness. Straining removes seeds for an ultra smooth mousse and the puree should be glossy and fluid, about one third of a cup. If puree seems watery the mousse can be runny, so press firmly through the sieve to concentrate the fruit. Avoid using under ripe or mealy berries since they will lack aroma and sweetness.
Place lemon juice and 1 tablespoon cold water in a small dish. Sprinkle gelatin over top and let sit 5 minutes until congealed.: Blooming the gelatin in cold water lets it swell and ensures even melting later, and you will see the gelatin granules absorb liquid and thicken into a glossy mass. This step is critical for a consistent set, so do not skip the five minute rest. If gelatin is not properly bloomed it can clump when heated and make the mousse grainy, which is a common pitfall to watch for.
Place finely chopped white or dark chocolate in a heat proof bowl.: Finely chopping your chosen chocolate allows it to melt quickly and smoothly when the warm cream is added, you should see a bed of small, even shards that will dissolve into a glossy ganache. Using larger pieces can require extra reheating and risks separation if overheated. To avoid seizing, ensure the bowl is dry and free of steam, because any stray water will cause the chocolate to become grainy.
Heat 1/3 cup of whipping cream in a small saucepan set over medium-low heat until it just barely begins to bubble (do not let it boil). Pour over chopped chocolate. Let sit for 30 seconds, then stir until smooth. If your chocolate was chopped very finely there will be more than enough residual heat to melt it; if not, microwave in 5 second intervals, stirring until just melted. Do not overheat or the mixture may separate.: As the heavy whipping cream warms you will smell a rich, dairy aroma and see the surface form tiny bubbles at the edge, a delicate simmer is the cue to stop heating. Pouring it over the chocolate and letting it sit allows gentle melting into a glossy emulsion, and stirring should produce a satiny, homogeneous mixture. Overheating can cause graininess or separation, so if additional heating is needed use short bursts and stir between each to preserve smoothness. A common error is boiling the cream which creates a dull, separated ganache rather than a lush base.
Microwave softened gelatin for 5 seconds to melt, then stir into melted chocolate mixture. Add raspberry puree and stir until incorporated.: Melting the bloomed gelatin briefly lets it dissolve into the warm chocolate base, and once combined the mixture should smell of both chocolate and bright fruit with a smooth, slightly thicker texture. Adding the strained raspberry puree transforms the color to a pleasant pink or rosy hue depending on your chocolate choice, and the mixture should blend without streaks. If you notice any gelatin lumps, pass the mixture through a fine sieve and remix; undissolved gelatin will create gritty pockets in the mousse.
In a large mixing bowl or the bowl of a stand mixer fitted with the whisk attachment, beat remaining 2/3 cup cream until frothy. Add powdered sugar and continue to beat until it forms medium-stiff peaks. Stir a spoonful of the whipped cream into raspberry mixture to lighten it. Fold in remaining whipped cream with a large rubber spatula until incorporated, pressing out any large lumps. At this point the mousse should hold medium peaks. If it seems runny, refrigerate for 10 to 20 minutes or until thickened (but do not refrigerate too long or it will set up completely). White chocolate mousse is more likely to need a bit of chilling than dark chocolate.: When whipping the remaining heavy whipping cream , watch for the stage where the foam becomes glossy and holds medium stiff peaks, a visual cue is that the peak tip stands but slightly curves at the end. Folding in a spoonful first lightens the dense puree and makes incorporation easier, then using a broad rubber spatula incorporate the rest with a gentle turning motion that preserves air. The mousse should feel silky and hold soft peaks; if it seems loose a short chill will help firm it without turning the texture chewy. Avoid over whipping which produces grainy, butter like bits that ruin the mousse’s silkiness.
Remove crusts from freezer. Spoon or pipe filling into individual cups, filling to within 1/4-inch of the rim of the wrappers. Top each with a whole raspberry. Refrigerate until set, at least 2 hours or overnight. Tarts will keep, lightly covered with plastic wrap, in the refrigerator for up to 2 days.: Filling chilled crusts prevents melting and helps the mousse settle with clean edges, and piping gives neat presentation while spooning works fine for a rustic look. Leave a small margin at the rim so the mousse does not overflow as it sets and place a reserved whole raspberry on each tartlet for an inviting garnish. As the mousse chills you will notice it firm from silky to sliceable, and when fully set it retains definition without weeping. A typical mistake is rushing the chill time which results in soft centers that lose shape when plated.
