1. Preheat oven to 350°F (175°C) and line an 8×8-inch pan with parchment paper.

2. Beat softened butter and granulated sugar until light and fluffy, about 2–3 minutes.

3. Add egg and vanilla extract, mixing until smooth.

4. Whisk together flour, baking powder, and salt in a separate bowl.

5. Gradually add dry ingredients to the wet mixture, mixing just until combined.

6. Press dough evenly into the prepared pan and bake for 18–22 minutes, until edges are lightly golden and center is soft.

7. Allow bars to cool completely in the pan on a wire rack.

8. Beat softened butter for frosting until creamy, about 1–2 minutes.

9. Gradually add powdered sugar, milk, and vanilla, mixing until smooth and adjusting consistency as needed.

10. Stir in food coloring if desired.

11. Spread frosting evenly over cooled bars and decorate with sprinkles if desired.

12. Lift bars from pan using parchment overhang and cut into squares with a sharp knife.