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 bowl, beat the softened butter and granulated sugar together using an electric mixer until pale, light, and fluffy, about 3 minutes.
03 - Add eggs one at a time, beating well after each addition. Stir in the vanilla extract until fully combined.
04 - In a separate bowl, whisk together the all-purpose flour, baking powder, and fine salt. Gradually add the dry mixture to the butter mixture, mixing on low speed until a soft, uniform dough forms.
05 - Split the dough evenly into three bowls. To one bowl, add the cocoa powder and mix until evenly incorporated for the chocolate layer. To the second bowl, add the strawberry extract and a few drops of pink or red gel food coloring, blending until the color is uniform for the strawberry layer. Leave the third bowl plain for the vanilla layer.
06 - 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 evenly. Finish by pressing the strawberry dough on top, smoothing the surface flat.
07 - Bake for 12 to 15 minutes until just set. Avoid overbaking to keep the layers soft and tender.
08 - Cool completely in the pan on a wire rack. Use the parchment overhang to lift the slab out, then cut into 36 bars or squares. Optionally, drizzle with melted semisweet chocolate and decorate with sprinkles or chopped nuts.