Preheat the oven to 350 degrees Fahrenheit (180 degrees Celsius) and line a metal cupcake pan with paper liners.
Add all purpose flour, cornstarch, granulated sugar, baking powder, and salt to the bowl of a stand mixer fitted with the paddle attachment. Mix on low until combined.
156 grams all purpose flour, 30 grams cornstarch, 200 grams granulated sugar, 2 teaspoons baking powder, ½ teaspoon salt
Add softened butter and beat on medium-low speed for 3-5 minutes. The mixture will look like wet sand.
56 grams unsalted butter
In a separate bowl, whisk the oil, eggs, vanilla extract, buttermilk, and whole milk together.
Mix the wet ingredients into the dry ingredients on low speed until the dry streaks of flour disappear. The batter will be very thin.
Use an ice cream or cookie scoop to evenly fill the cupcake liners ½ to the top. Bake for 16-18 minutes or until a toothpick inserted in the center of a cupcake comes out clean with a few moist crumbs.
Let the cupcakes cool in the hot pan for 5-10 minutes. Carefully transfer to a wire cooling rack. Cool completely before assembling.
Microwave the dulce de leche filling for 20 seconds to loosen it. Then cut out the cupcake centers with a piping tip or cupcake corer. Fill each cupcake with 1-2 teaspoons of dulce de leche.
230 grams dulce de leche
Frosting
Beat softened butter with a mixer on medium-high speed until smooth and creamy. Then beat in the dulce de leche on medium speed.
226 grams unsalted butter, 150 grams dulce de leche
Scrape down the sides of the bowl with a rubber spatula and add the powdered sugar one cup at a time. Mix on low speed, then add the vanilla and salt. Increase to medium speed until light and fluffy.
360 grams powdered sugar, 2 teaspoons vanilla extract, ½ teaspoon salt
Add the buttercream to a piping bag filled with a large piping tip. Frost the cupcakes, garnish with flaky salt or sprinkles, and enjoy!
Notes
Room temperature ingredients: Make sure your dairy ingredients and eggs are room temperature so they are easy to mix. Pull them out of the fridge 1-2 hours before baking.Measure the flour in grams: It's important to measure the flour in grams with a kitchen scale for the most accurate results. If you don't have one, fluff up the flour in the bag. Gently spoon it into your measuring cup (without packing it in) until just overflowing. Swipe the excess off into the bag with a butter knife.