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 with an electric mixer until pale, light, and fluffy, about 3 minutes.
03 - Add the eggs one at a time, beating well after each addition. Scrape down the bowl as needed, then mix in the vanilla extract until combined.
04 - In a separate medium 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, mixing on low speed until a soft, uniform dough forms. Avoid overmixing.
06 - Split the dough evenly into three separate bowls. Add the cocoa powder to the first bowl and mix thoroughly for the chocolate layer. Add the strawberry extract and a few drops of pink or red gel food coloring to the second bowl and blend until evenly tinted for the strawberry layer. Leave the third bowl plain for the vanilla layer.
07 - Press the chocolate dough evenly across the bottom of the prepared pan, smoothing with an offset spatula. Carefully spread the vanilla dough over the chocolate layer, smoothing evenly. Finally, press the strawberry dough on top, smoothing the surface flat.
08 - Bake for 12 to 15 minutes, or until the top is just set and the edges are lightly golden. Do not overbake.
09 - Let the cookie slab cool completely in the pan set on a wire rack. Use the parchment overhang to lift it out, then cut into 36 bars or squares. Optionally, drizzle with melted semisweet chocolate and finish with sprinkles or chopped nuts.