Preheat oven to 350°F and line an 8x8 baking pan with parchment paper or foil.
In a large bowl, cream together butter, granulated sugar, and brown sugar until smooth and creamy.
Add the egg, vanilla extract, and almond extract to the butter mixture and mix until combined.
Gradually mix in the flour, baking powder, cornstarch, and salt just until the flour disappears.
Spread the batter evenly into the prepared baking pan and bake for 15 minutes; the center may look slightly raw.
Meanwhile, prepare the frosting by whipping softened butter and powdered sugar until combined.
Add heavy cream and vanilla extract to the frosting and whip until light and fluffy.
Once bars have cooled, spread the frosting evenly on top and add Christmas sprinkles.
Slice and serve immediately for softer frosting, or chill to set the frosting before serving.