Bring a tropical wave to your winter with these holiday cookie swap-ready triangles that are chewy, crunchy, and absolutely delightful.
Preheat oven to 350 degrees. In a medium bowl, beat together butter, 1 cup brown sugar, and 2 cups flour until mixture comes together to form a dough. Press evenly into a rimmed 11-by-17- inch baking sheet. Bake until just golden, about 15 minutes. Remove from oven; let cool slightly.
Meanwhile, pulse coconut in a food processor several times, until coarsely chopped, then transfer a large bowl. Stir in remaining 2 cups brown sugar and 1/4 cup flour along with baking powder, eggs, vanilla, and nuts. Spread mixture evenly over cooled crust.
Bake, rotating sheet halfway through, until crust is golden brown and filling is set, about 20 minutes. Remove from oven; transfer to a wire rack to cool completely. Cut into triangles with a serrated knife. Store up to 2 days in an airtight container at room temperature.
You can bake the crust a day ahead, and keep it covered at room temperature.