1. Preheat the oven to 350°F (175°C) and line a 10-inch cake pan with parchment paper.
2. In a large mixing bowl, cream together butter, brown sugar, sugar, and vanilla until light and fluffy (about 3 minutes).
3. Add the egg and egg yolk, mixing well after each addition.
4. In a separate bowl, whisk together flour, cornstarch, baking powder, and salt.
5. Gradually add the dry ingredients to the wet ingredients, mixing just until combined.
6. Fold in chocolate chips and sprinkles with a spatula.
7. Transfer the dough to the prepared pan, spreading it evenly.
8. Bake for 18-22 minutes, or until the edges are golden brown and the center is set.
9. Cool completely in the pan before removing.

To Make the Vanilla Buttercream Frosting:

1. Beat butter on medium speed until smooth and creamy.
2. Gradually add powdered sugar, vanilla, and salt, mixing until combined.
3. Adjust the frosting’s consistency by adding 1-2 tablespoons of milk or cream.
4. Transfer frosting to a piping bag for decorating.

Decorating and Serving:

• Pipe frosting around the edges or write a message on top.
• Add extra sprinkles, chocolate drizzle, or heart-shaped candies for a festive touch.