01 - Preheat oven to 350°F. Line a 9 x 13 inch baking pan with parchment paper, leaving 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 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. Avoid overmixing.
06 - Split the dough evenly into three separate bowls. To the first bowl, fold in the cocoa powder until uniformly blended for the chocolate layer. To the second bowl, add the strawberry extract and a few drops of pink or red gel food coloring, mixing until the color is even for the strawberry layer. Leave the third bowl uncolored 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, smoothing to an even thickness. Finally, press the strawberry dough over the vanilla layer and smooth the top surface.
08 - Bake for 12 to 15 minutes until the top is just set and the edges are lightly golden. Do not overbake to keep the layers tender.
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.