Preheat the oven to 350 degrees F (175 degrees C).
STEP 2
Beat butter and 1/2 cup brown sugar until smooth. Stir in 1 1/2 cups flour. Press firmly into the bottom of a 9x13-inch pan.
STEP 3
Bake until crust is golden, about 15 minutes.
STEP 4
Combine remaining 1 1/2 cups brown sugar and beaten eggs. Stir in vanilla. Whisk remaining 1/4 cup flour, cocoa powder, baking powder, and salt together; stir into the egg mixture.
STEP 5
Fold in coconut and walnuts; pour into crust.
STEP 6
Bake until firm, 25 to 30 minutes. Cool, then cut into squares.