Preheat the oven to 350°F. Line a 9×9-inch baking pan with parchment paper.
In a large bowl, beat the softened butter and granulated sugar until light and fluffy, about 2 minutes.
Mix in the egg, egg yolk, vanilla extract, and almond extract, if using.
Add the flour, cornstarch, baking powder, and salt. Mix until a soft dough forms.
Press the dough evenly into the prepared baking pan.
Bake for 22–27 minutes, or until the edges are lightly golden and the center is set. Avoid overbaking for soft bars.
Let the bars cool completely before frosting.
Beat the softened butter until creamy. Add the powdered sugar, vanilla, salt, and 1 tablespoon of milk or cream. Beat until light and fluffy, adding more milk as needed.
Spread the frosting evenly over the cooled bars and decorate with Christmas sprinkles.
Slice into 16 squares and serve.
