Preheat the oven to 350 degrees F (175 degrees C). Grease an 8x12-inch baking pan.
Beat butter, 1/2 cup brown sugar, and 1 cup flour together in a bowl using an electric mixer until well blended. Press into the prepared pan.
Bake in the preheated oven until firm but not browned, about 20 minutes. Remove from oven and reduce temperature to 325 degrees F (165 degrees C).
Beat 1 cup brown sugar, eggs, 2 tablespoons flour, vanilla extract, baking powder, and salt together in a bowl using an electric mixer until mixture is light, about 2 minutes. Add walnuts; beat until blended but not pureed. Spread mixture over the crust; sprinkle with coconut.
Bake in the preheated oven until set, about 25 minutes. Cut while warm.