Preheat your oven to 350°F. Line an 8×8-inch baking pan with parchment paper.

In a large mixing bowl, cream the softened butter and granulated sugar together until the mixture looks light and smooth. Add the egg and vanilla.

Add the flour, baking powder, and salt. Mix just until a soft dough forms. Try not to overmix here, since that can make the bars less soft. Gently fold in the sprinkles.

Press the dough evenly into your prepared pan. The dough will be thick, so I like to use clean hands or the back of a spoon to help smooth it out into the corners.

Bake for 18 to 22 minutes, or until the center looks set and the edges are just lightly golden. You do not want to overbake these if you want that soft sugar cookie bar texture.

Let the bars cool completely in the pan before frosting.

To make the frosting, beat the softened butter until smooth. Add the powdered sugar, vanilla extract, and 1 tablespoon of milk. Beat until creamy. If needed, add a little more milk until the frosting is spreadable.

Spread the frosting over the cooled bars, then top with extra sprinkles. Slice into squares and serve.