Prep: Preheat your oven to 350°F (175°C) and line a 9x9” baking pan with parchment paper, leaving an overhang for easy removal.
Cream Butter & Sugar: In a large bowl, cream the softened butter and sugar together using a hand mixer on medium speed until light and fluffy (about 2–3 minutes).
Wet Ingredients: Beat in the room-temperature egg and vanilla extract until just combined.
Dry Ingredients: In a separate bowl, whisk together the all-purpose flour and baking powder.
Combine: Slowly add the dry ingredients to the wet mixture and mix on low speed until just combined. Do not overmix.
Sprinkles: Gently fold in the 1/4 cup of patriotic sprinkles using a silicone spatula.
Bake: Spread the thick dough evenly into the prepared pan. Bake for 20–22 minutes, or until lightly golden at the edges. Do not overbake. Let the bars cool completely to room temperature in the pan.
Make Frosting: To make the frosting, beat together the softened butter and softened cream cheese until smooth. Add the powdered sugar, vanilla, and milk. Beat on high speed until smooth, fluffy, and spreadable.
Decorate: Use the parchment overhang to lift the cooled bars out of the pan. Spread the frosting in an even layer over the bars and top generously with more patriotic sprinkles.
Chill & Serve: Slice into squares. For the absolute best texture and flavor, store the bars in an airtight container in the refrigerator and serve them COLD!