Banana Cake with White Chocolate Macadamia Frosting

Name: Banana Cake with White Chocolate Macadamia Frosting
Ingredients: 1 3/4 teaspoons baking powder; 2 tablespoons banana pudding mix; 1 cup white sugar; 1/2 cup buttermilk; 1 ripe banana, mashed; 1/2 cup butter; 2 eggs; Buttercream Frosting, recipe follows; 2 teaspoons vanilla extract; Macadamia nuts, dry roasted and finely chopped; 1 1/2 cups all-purpose flour; 1/2 cup (1 stick) butter, at room temperature; 2 to 3 cups confectioners' sugar; 1/8 to 1/4 cup melted white baking chocolates; 1/2 teaspoon vanilla extract