The dark, nutty butter gives these squares a deeper, richer flavor. Martha made this recipe on Martha Bakes episode 609.
Preheat oven to 350 degrees. Butter a 9-by-13-inch baking pan, and line with parchment paper. Butter and flour parchment paper.
Cook butter in a saucepan over medium heat until it turns golden-brown; remove from heat, transfer to a large bowl and let cool. Add sugars and stir until combined. Add eggs and vanilla; beat to combine. Add flour, baking powder, salt, cashews, and coconut. Mix until thoroughly combined, and transfer to prepared pan.
Bake until a cake tester inserted in the center just comes out clean, 35 to 40 minutes (do not over bake). Transfer to a wire rack to cool completely before turning out of pan onto a cutting board. Peel off parchment paper; cut blondies into 3-inch squares.
Blondies can be stored in an airtight container at room temperature for up to 3 days.