Grease a 15x10x1-inch baking pan; set aside.
In a large mixing bowl, beat the butter with an electric mixer on medium to high speed about 30 seconds. Beat in the sugar, baking powder, baking soda, and salt until combined. Beat in the egg, mashed bananas, sour cream, and vanilla. Beat or stir in the flour. Stir in the toffee pieces and almonds, if you like.
Pour the batter into the prepared baking pan, spreading evenly. Bake in a 350 degree F oven about 25 minutes or until a wooden toothpick inserted near center comes out clean. Cool in pan on wire rack.
Prepare the Brandied Brown Butter Frosting and spread over cooled banana bars. Makes 48 bars.
In a small saucepan heat butter over low heat until melted. Continue heating until the butter turns a light brown. Remove from heat; pour into a medium bowl. Add powdered sugar, brandy or milk, and vanilla. Beat with an electric mixer on low speed until combined. Beat on high to medium speed, adding enough milk to make frosting of spreading consistency. Use immediately.