Preheat oven to 350 degrees F. Line an 8 X 8 X 2-inch baking pan with foil, extending the foil up over the edges of the pan. Divide cookie dough and place in prepared pan. Using your hand, press dough into an even layer on the bottom of the pan.: As you press the triple chocolate cookie dough into the foil lined pan, you will feel it become more compact and slightly tacky under your palms, releasing a rich chocolate fragrance that tells you the oven is about to reward you. Pressing by hand gives you the best control over thickness, and aim for an even surface so baking is uniform. If the dough resists spreading, let it sit at room temperature for a few minutes to relax; forcing cold dough can tear the foil or create air pockets. A common mistake at this stage is uneven pressing, which leads to thin edges and a thicker center, so smooth the corners carefully and check the edges with your fingertips to make sure the layer is consistent.
Bake for 18 minutes. Immediately sprinkle with the marshmallows and half of the chocolate pieces. Bake for 3 to 4 minutes more or until marshmallows are puffed, but not browned. Sprinkle with crushed peppermint candies. Cool completely.: When the pan comes out after the initial 18 minutes, the dough will be set but still soft in the center, and steam may rise as you open the oven, releasing a warm chocolate perfume. Scatter the tiny marshmallows and half of the semisweet chocolate pieces across the top so they melt into pockets, then return to the oven briefly until the marshmallows puff and turn glossy. Watch closely during this short interval, because marshmallows can go from puffed to browned quickly; remove them when they are tall and pillowy but still white at the edges. Immediately sprinkle the crushed striped round peppermint candies or candy canes so they stick to the soft marshmallow surface and retain some crunch. Let the pan cool fully on a rack, because slicing while warm will cause the marshmallow to pull and the toppings to slide.
To serve, remove from pan by lifting foil. Place on a cutting board. Using a long, sharp knife, cut into squares. In a small saucepan, heat and stir the remaining chocolate pieces and the shortening over low heat until melted. Drizzle over brownies. Let stand until chocolate is set. Makes 25 brownies.: Lifting the foil will reveal a glossy top with pockets of white marshmallow and specks of crushed peppermint. Place the foil block on a cutting board, and use a long, sharp knife for clean cuts; the goal is sharp edges without dragging the marshmallow into strings. For the drizzle, place the reserved semisweet chocolate pieces and shortening in a small saucepan over low heat, stirring gently until smooth and shiny. The shortening helps the melted chocolate flow and set with a glossy sheen. Drizzle with a spoon or fork in sweeping motions so each square gets a finishing touch. One mistake is applying the drizzle while the brownies are too warm, which causes it to sink in and lose its decorative look, so wait until the bars are mostly cool before drizzling.
Test Kitchen Tip:: : Between cuts run your knife under hot water and pat with a paper towel to dry. : A hot knife slices through the marshmallow layer with minimal pulling, leaving clean edges and tidy squares. Heat the blade under hot running water for a few seconds, then wipe it dry before each cut so the heated metal glides through the marshmallow and chocolate. The sensory cue here is smooth resistance then a clean glide as you slice. A trap to avoid is using a dull knife, which squashes the bars rather than slicing them, so sharpen or use a long chef's knife for best results.
A hot knife will more easily cut through the marshmallows. Between cuts run your knife under hot water and pat with a paper towel to dry.: : Place cookies in layers separated by waxed paper in an airtight container; cover. Store at room temperature for up to 3 days. : When you stack the cooled squares with waxed paper between layers, the marshmallow and drizzle stay intact and the peppermint retains crunch. At room temperature the texture remains pleasing for several days, but refrigeration can make the marshmallow firmer and the chocolate more set. A frequent misstep is sealing still warm bars, which creates condensation and softens textures, so ensure they are completely cool before storing.
To Bake Ahead:: For Christi's version, replace semisweet chips with mint chocolate chips. Finish with decorative sprinkles. : Swapping in mint chocolate chips intensifies the mint profile and offers a slightly sweeter, creamier drizzle when melted. If you try this variation, note that different chips melt with different viscosities, so stir gently and keep heat low to maintain a glossy finish. Overheating mint chips can break the cocoa butter and change texture, so patience is key.
