Preheat oven to 350ºF (177ºC). Line the wells of a 12-count cupcake pan with cupcake liners and set aside.
In a large bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, beat together the melted butter, brown sugar, egg, milk/cream, yogurt, and vanilla extract on medium to high speed until completely smooth.
Reduce the mixer speed to low, then slowly add the dry ingredients to the wet ingredients until just combined. Then, fold gently by hand with a large spatula and press out any large lumps of flour that remain by scraping them against the side of the bowl. Small lumps are ok.
Fill the cupcake liners ⅔ to ¾ of the way full with batter. Bake for 15-17 minutes or until a toothpick inserted in the center comes out clean. Allow to cool completely before decorating.
In a large bowl with a handheld mixer, or a stand mixer fitted with the paddle attachment, beat the shortening on medium speed until creamy, about 2 minutes.
With the mixer on low, add in the powdered sugar, dulce de leche, and vanilla. Increase mixer speed to medium and beat until completely smooth, about 3 minutes. Add milk or cream if frosting seems to need to be thinned out. Add salt to taste.
Using a melon baller, the bottom of large piping tip, or a small paring knife, remove part of the middle of the top of each cupcake about a nickel size diameter and about ½" depth. Set the piece of cupcake aside.
Fill each cupcake with about 1 teaspoon of dulce de leche sauce and replace the reserved piece of cupcake. If some spills out, that's ok, but press gently to avoid excess spillage.
Frost the cupcakes using a piping bag or a large zip top bag with the corner snipped off. If desired, press a small offset spatula into the top of the mound of frosting to create a well for more dulce de leche sauce. You can also just drizzle the cupcakes with dulce de leche sauce, if preferred. Cupcakes will stay fresh covered at room temperature up to 3 days. Store in the refrigerator up to 5 days. Cupcakes freeze well. Do not fill or frost until cupcakes are thawed and you are ready to serve. Thaw cupcakes at room temperature.
Room temperature ingredients: these are always a good idea when using room temperature butter as it makes for easier incorporation into the batter. To bring eggs to room temperature quickly, place eggs in a bowl of warm water for 5-10 minutes.
Full fat dairy: it is imperative to use full fat dairy in this recipe for moisture, since we’re using melted butter, therefore removing some natural moisture.
Dulce de leche: you can use homemade or store bought. Textures/viscosities can vary depending on the method you make it or the brand you purchase. Looser dulce de leche will be messier than a thicker texture. Keep this in mind when making or purchasing your sauce.
Vegetable shortening: I chose vegetable shortening for this dulce de leche buttercream because the sauce I used was very thin. If your sauce is thicker and sturdier, you can use room temperature butter in place of the shortening.
Milk or cream: depending on the viscosity of your dulce de leche sauce, you may not need any additional liquid to thin out your buttercream. Add liquid with caution.