Preheat oven to 350° degrees F. Line a 9x13 light metal rectangular baking pan with parchment or foil, extending the sides of the parchment or foil over the edges of the pan. Spray lightly with cooking spray; set aside.
For the cookies: In the bowl of a stand mixer, cream together the butter and granulated sugar until smooth and fluffy, about 1 minute. Beat in the eggs, one at a time, mixing well after each addition and scraping down the sides and bottom of the bowl as needed, followed by the vanilla extract and almond extract or citrus zest, if using.
Lastly, add in the baking powder, cornstarch, and flour and mix on low speed until just barely incorporated. Do not overmix. Spread the batter into the prepared pan in an even layer.
Bake for 14-16 minutes or until just barely browned. It will still look slightly underdone and very pale; this is normal and remember, it will continue finishing cooking in the hot pan outside of the oven. Cool completely, about 90 minutes.
For the vanilla buttercream: In the bowl of a stand mixer, cream together the butter and vanilla until whipped and combined, about 30 seconds. Alternate adding in the confectioners' sugar and heavy cream, mixing well after each addition until frosting is light and fluffy.
Spread the frosting evenly over the sugar cookie bars; top with sprinkles, if using.