1. Preheat the oven to 350°F (175°C) and line a 9×13-inch baking pan with parchment paper, leaving an overhang.

2. In a large bowl, beat 1 cup softened butter and 1 cup granulated sugar until light and fluffy, about 2 to 3 minutes.

3. Add the eggs one at a time, then stir in 2 teaspoons vanilla extract until fully combined.

4. In a separate bowl, whisk together 2 1/2 cups flour, 2 teaspoons cornstarch, 1 teaspoon baking powder, and 1/2 teaspoon salt.

5. Gradually add the dry ingredients to the butter mixture on low speed and mix until just combined to avoid overmixing.

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

7. Bake for 18 to 22 minutes, until the edges are lightly golden and the center is set but still soft.

8. Allow the bars to cool in the pan for 10 minutes, then lift out with the parchment paper and cool completely on a rack.

9. For the frosting, beat 1/2 cup softened butter until creamy, then gradually add 2 1/2 to 3 cups powdered sugar and 2 to 3 tablespoons cream until smooth; stir in 1 teaspoon extract and optional food coloring.

10. Once the bars are fully cool, spread the frosting evenly, add sprinkles if desired, then slice into squares and serve.