01 - Preheat oven to 350°F. Line a 9×13-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a large mixing bowl, beat the softened butter and granulated sugar together using an electric mixer on medium-high speed until pale, light, and fluffy, about 3 to 4 minutes.
03 - Add the eggs one at a time, beating well after each addition until fully incorporated. Scrape down the bowl as needed, then mix in the vanilla extract.
04 - In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the butter mixture on low speed, mixing just until a soft, cohesive dough forms.
05 - Split the dough evenly into three portions. To the first portion, fold in the cocoa powder until uniformly colored for the chocolate layer. To the second portion, add the strawberry extract and a few drops of gel food coloring, blending until evenly tinted pink for the strawberry layer. Leave the third portion plain for the vanilla layer.
06 - Press the chocolate dough evenly across the bottom of the prepared pan, smoothing with an offset spatula. Gently spread the vanilla dough over the chocolate layer, smoothing to an even thickness. Finish by spreading the strawberry dough on top, smoothing the surface completely flat.
07 - Bake on the center rack for 12 to 15 minutes, until just set and the top layer is no longer wet-looking. Do not overbake to keep the cookies tender.
08 - Cool completely in the pan set on a wire rack. Use the parchment overhang to lift the slab out onto a cutting board. Cut into 36 bars or squares. Optionally, drizzle with melted semisweet chocolate and finish with sprinkles or chopped nuts.