Preheat your oven to 375 degrees. This step is crucial for the perfect bake. While the oven warms up, line a 9×13-inch baking pan with foil and lightly grease it. This will make removing the bars much easier once they're baked.
In a large bowl, using an electric mixer, beat butter and granulated sugar together until the mixture is light and fluffy, which should take around 2 minutes. The texture should be airy, indicating that air has been incorporated.
Add the egg, sour cream, and both extracts to the bowl. At this point, it's normal for the mixture to look curdled. Don’t worry; it will come together in the next step!
In a separate bowl, combine the flour, baking powder, and salt. With your mixer on low, gradually add this flour mixture to the wet ingredients, mixing just until everything is incorporated. You don’t want to overmix—just blend until the flour is no longer visible.
Transfer the dough into your prepared baking pan. Use your hands or a spatula to press it evenly into the pan. It should be smooth and even.
Bake in the preheated oven for about 15 minutes. The bars will look slightly undercooked when you take them out, but they'll continue cooking while cooling. Trust me, this will ensure they stay soft!
Once baked, allow the bars to cool in the pan. Meanwhile, let's prepare the frosting: beat together the butter and powdered sugar until well mixed. Add the extracts and milk, beating until smooth and creamy.
Spread the frosting generously over the cooled bars. Feel free to make it as thick or thin as you like.
Finally, sprinkle your sprinkles on top, creating a fun, colorful finish. Cut into squares or rectangles and serve!