These sugar cookie bar cookies are deliciously frosted and perfect for any occasion! Customize them with the frosting and sprinkle colors of your choice.
Preheat the oven to 350°F/180°c. Line an 8-inch square baking tin with foil or parchment paper. Set aside.
Add the softened butter and the granulated sugar to the bowl of a stand mixer. Beat on a medium-high speed using the flat beater attachment for 3 minutes, or until pale and creamy.
Add the eggs and vanilla extract to the butter mixture and beat until well combined, scraping the sides of the bowl as necessary.
Add the flour, baking powder, salt, and cornstarch to the bowl. Beat on a low speed until just combined.
Gently fold through the sprinkles until combined.
Spoon the mixture into the prepared baking tin and press down. Bake for 20 minutes, or until just starting to turn golden. Allow to cool completely in the tin.
To make the frosting, add the softened butter to a clean stand mixer bowl and beat on a medium-high speed using a flat beater attachment for 2 minutes, or until pale and creamy. Add the powdered sugar in 2 batches, beating well after each addition, starting on a low speed and gradually increasing to a high speed.
Add the vanilla extract, salt, and food coloring (if using) to the butter mixture and beat until well combined.
Spread the frosting over the cooled cookie bars.
Garnish with the sprinkles and slice before serving.