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

2. In a bowl, beat softened butter and granulated sugar until light and fluffy, about 2 minutes.

3. Add egg and vanilla extract and mix until combined, scraping down bowl sides as needed.

4. In a separate bowl, whisk flour, cornstarch, baking powder, and salt.

5. Gradually add dry ingredients to wet ingredients and mix until just combined without overmixing.

6. Press dough evenly into the prepared pan using a spatula or lightly floured hands.

7. Bake for 18–22 minutes until edges are lightly golden and center is set.

8. Cool in pan for 10 minutes, then lift out with parchment and transfer to a cooling rack to cool completely.

9. For frosting, beat softened butter until creamy, then gradually add powdered sugar and milk until smooth and spreadable.

10. Divide frosting and tint with food coloring to desired pastel shades.

11. Spread frosting over cooled bars and smooth with an offset spatula.

12. Let frosting set at room temperature for 15–20 minutes before slicing into bars.