01 - Preheat oven to 350°F. Line a 9 x 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 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 all-purpose flour, baking powder, and fine salt until evenly distributed.
05 - Gradually add the flour mixture to the butter mixture on low speed, mixing just until a soft, cohesive dough forms. Do not overmix.
06 - Divide the dough into three equal 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 gel food coloring, blending until evenly pink for the strawberry layer. Leave the third portion plain for the vanilla layer.
07 - Press the chocolate dough evenly into the bottom of the prepared pan, smoothing with an offset spatula. Carefully spread the vanilla dough over the chocolate layer. Finish by pressing the strawberry dough on top, smoothing the surface to an even layer.
08 - Bake for 12 to 15 minutes until the top is just set and the edges are lightly golden. Do not overbake to maintain a tender, chewy texture.
09 - Cool completely in the pan 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 decorate with sprinkles or chopped nuts.