Preheat oven to 375°F/190°C. Line a 9x13 baking dish with foil and lightly spray foil with nonstick cooking spray.
In a medium bowl, whisk together the flour, baking powder, and salt.
In a large bowl, beat the butter and sugar until pale and fluffy, about 3 or 4 minutes. Stir in the egg and egg white. Add sour cream, vanilla, and almond extract (if using) and mix until blended. Slowly add flour mixture and stir until combined. With buttered hands, gently press and pat the batter into the prepared pan.
Bake for 15 to 17 minutes, until a toothpick inserted in the center comes out clean and the bars are set. The bars will still be pale, but the edges may start to turn golden brown. Cool completely, then remove from pan, frost, and cut into squares.
Frosting: In a medium bowl, beat the butter and cream cheese together until light and creamy, about 2 minutes. Mix in the vanilla. Add the powdered sugar gradually and mix on low speed after each addition, until well blended and creamy. Add the heavy cream and whip until frosting is light and fluffy.
Notes
To make these bars in a large rimmed cookie sheet, do 1 1/2 times the ingredients in the recipe.