In a large bowl, cream together the butter and sugar until light and fluffy. Add the egg, vanilla, and almond extract (if using), and mix until well combined.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture and mix until a soft dough forms.
Preheat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper or lightly grease it. Press the dough evenly into the pan. Bake for 18–22 minutes, or until the edges are lightly golden. Let cool completely before frosting.
In a medium bowl, beat the softened butter until creamy. Gradually mix in the powdered sugar, vanilla, and cream or milk. Add food coloring of choice and beat until fluffy and smooth. Spread evenly over cooled cookie bars and top with festive sprinkles.