For the Cupcakes:
3 cups all-purpose flour
1/4 cup cornstarch
1 1/2 teaspoons baking powder
1 1/2 teaspoons baking soda
1 teaspoon salt
3/4 cup (1 1/2 sticks) unsalted butter, softened
1 cup brown sugar
1/2 cup granulated sugar
1 teaspoon pure vanilla extract
3 large eggs
1 1/2 cup buttermilk

For the Frosting:
1/2 cup (1 stick) unsalted butter, softened
8 ounces cream cheese, softened
3/4 cup dulce de leche, plus more for drizzling
1/4 teaspoon salt
2 to 3 cups powdered sugar