Preheat the oven to . Lightly spray an 8x8-inch baking dish with nonstick spray.
Whisk together the all-purpose flour, buckwheat flour, cocoa powder and baking powder.
Use an electric mixer to beat together the sugar, egg whites, canola oil, prunes and vanilla extract.
Stir the flour mixture into the sugar-and-egg mixture until fully incorporated.
Pour the brownie batter into the prepared baking dish and bake in the preheated oven for 25 minutes.
Meanwhile, melt the butter in the microwave for 30 seconds. Whisk in the other icing ingredients until smooth.
Pour the icing on top of the brownies. Arrange the candy on top of the brownies before cutting into 16 equal squares.