In the bowl of the stand mixer, cream the butter. Gradually add the brown sugar and beat until smooth. Stir in the flour until the mixture is smooth. Press the dough into the bottom of the square pan. Bake in the preheated oven for 20 minutes.
While the crust bakes, clean the bowl and paddle. In the small bowl, toss the shredded coconut with the flour. In the bowl of the stand mixer, beat the eggs and brown sugar until smooth. Stir in the vanilla, chopped walnuts, and the shredded coconut mixture.
Spread the batter over the baked crust and bake for 20 minutes longer. Cool completely. To serve, sprinkle with powered sugar and cut into 24 squares or bars.