Preheat the oven to 350°F. Place 12 cupcake liners in a 12-count muffin tin. Set aside.
In a large bowl, beat the sugars and butter until light and creamy. Add eggs and vanilla, and beat again.
¾ cup (150g) granulated sugar, ½ cup (100g) light brown sugar, ½ cup salted butter, 2 large eggs, 2 teaspoons pure vanilla extract
Stir flour, cornstarch, baking powder, salt, and baking soda in another bowl.
1 ½ cups (212g) all-purpose flour, 3 Tablespoons cornstarch, 1 teaspoon baking powder, ½ teaspoon salt, ¼ teaspoon baking soda
Add dry ingredients, mixing until combined.
Stir in sour cream and milk just until mixed.
½ cup sour cream, ¼ cup whole milk
Divide batter evenly between cupcake liners filling each 2/3 full.
Bake for 20-22 minutes. Remove and cool.
In a separate large bowl, beat butter until light. Add powdered sugar, ⅔ cup of the dulce de leche, and salt. Beat until creamy and smooth.
½ cup salted butter, 4 cups (480g) powdered sugar, 1 (13.4-ounce) can dulce de leche, ¼ teaspoon salt
Use a cupcake corer to remove the center of each cupcake. Fill each center with 1 teaspoon of the dulce de leche.
Pipe or spread remaining dulce de leche in a line on a large piece of plastic wrap. Spoon frosting next to the line of dulce de leche, then roll up plastic wrap to create a log, twisting the ends to hold in the frosting.
Transfer to a piping bag fitted with a wide, round piping tip, pulling the end of plastic wrap through and snipping it off so the frosting can pipe through.
Pipe swirls of frosting onto each cupcake.