Preheat oven to 350ºF.: When you set the oven to 350ºF , you should notice a gentle warmth spreading through the kitchen, and a faint dry scent from the heating element. Preheating is essential so the crust begins to bake immediately, creating that slightly toasted texture; without a preheated oven the base can absorb moisture and become soggy. A common mistake is not waiting until the oven reaches temperature, which changes baking time and texture. Listen for the oven click or check with an oven thermometer to be sure it is accurate.
Line an 8 x 8 pan with foil.: Lining an 8 x 8 pan with foil gives you a smooth release and makes lifting the bars simple after chilling. The foil will shine and crinkle under your fingers as you press it into corners, and it helps protect edges from overbrowning. Not lining the pan can make removal difficult and risk breaking the squares. Make sure the foil is pressed into corners without gaps for neat slices.
Mix first 3 ingredients in medium bowl.: As you combine the crushed NILLA Wafers , flour , and packed brown sugar , the bowl will fill with a toasty, sweet aroma. The visual cue is a uniformly colored crumb, and the feel should be slightly sandy. This blending ensures even flavor distribution so every bite of crust tastes the same. Skipping thorough mixing can leave pockets of dry flour or sugar, causing inconsistent texture.
Cut in butter until mixture resembles coarse crumbs. Press the crumbs onto the bottom of the prepared pan and bake for 15 minutes.: You will notice the cold butter turning into small pea sized pieces as you work it in, making the mixture look like coarse meal. Pressing these crumbs firmly into the lined pan creates a compact base that holds together once baked. As it bakes for 15 minutes the kitchen will be filled with a warm, slightly caramelized scent, and the edges may deepen to a light golden color. Be careful not to press too thinly, or the crust can burn at the edges, and avoid leaving large butter chunks that prevent even baking.
While the crust is baking, beat cream cheese and granulated sugar with mixer until well blended. Add eggs one at a time and 2 Tbsp. flour; mix well.: The sound of the mixer changes as the cream cheese and granulated sugar transform into a smooth, glossy mixture. Initially it will look thick and slightly grainy, then become silky as you beat. Adding the eggs one at a time helps the mixture emulsify, creating a stable filling, while the extra flour stabilizes the custard. If you add eggs too quickly you risk curdling, and over beating after eggs can incorporate excess air that leads to cracking during baking.
Blend in 1 Tbsp. zest, juice and baking powder; pour over crust.: When you fold in the reserved lemon zest , fresh lemon juice , and baking powder , a bright citrus perfume will rise. The batter will loosen slightly and shine, ready to pour over the warm crust. The acid from the juice interacts with the batter to create a silky, tangy profile. If the crust is too hot it can cause the filling to start setting unevenly, so let the crust cool briefly after the first bake to avoid curdling the filling when poured.
Bake for 25 to 28 min. or until center is set. Let cool completely then refrigerate for 2 hours. Sprinkle with powdered sugar and remaining zest before serving.: As the bars bake, the edges will puff and take on a pale golden hue while the center firms. The visual cue is a gentle jiggle at the center that dissipates when lightly shaken, and the aroma becomes a balanced tart and sweet note. This timing is critical because under baking yields a runny center and over baking makes the filling rubbery. To avoid this, check at the earliest time and rely on that faint jiggle rather than strictly on a timer.
Let cool completely then refrigerate for 2 hours.: Cooling allows residual heat to finish setting without over baking, and refrigeration for 2 hours firms the filling so the squares slice cleanly. You will notice the texture tighten and the lemon scent mellow slightly during chilling. A typical error is trying to cut them warm, which leads to messy, collapsing pieces. Patience here guarantees tidy, pretty servings.
Sprinkle with powdered sugar and remaining zest before serving.: Dusting the chilled bars with powdered sugar creates a delicate, snowy finish and the reserved zest gives a fresh visual pop and tiny reawakened aroma. This final touch balances appearance and flavor. If you sugar too early or while bars are warm, the sugar will dissolve into moisture and lose the crisp look, so wait until fully chilled.
