Preheat your oven to 350F. Prepare your pan by lining with cupcake liners and set aside.
In a medium-sized bowl, sift flour, baking powder and salt. Set aside.
Using a handheld electric mixer or a stand mixer fitted with the paddle attachment, beat butter and sugar on medium-high speed until light and fluffy.
Switch to low speed and add the egg. Once incorporated well, add the egg white and the vanilla extract
Still on low speed, add a third of your flour mixture then half of your milk. Add another third of your flour, the rest of your milk and finally the rest of your flour. Do not over-beat; just mix until each addition is incorporated and no significant traces of flour can be seen. Remember to scrape the bottom and sides of your bowl.
Scoop the batter into your cupcake liners (no more than ¾ full) and bake for 18-20 minutes or until a toothpick inserted in the centre of a few cupcakes comes out clean.
Take out of the oven and let cool in the pan on a wire rack for about 5 minutes. Then take out of the pan and cool the cupcakes on the wire rack completely. In the meantime, you can make your frosting.
For the frosting
In a large bowl using a handheld electric mixer or using a stand mixer fitter with the paddle attachment, beat softened butter until very light and fluffy (about 10 minutes).
Add dulce de leche and a pinch of salt. Beat until incorporated. Scrape the bottom and sides of your bowl.
Continue to beat until the frosting is light, fluffy and you get the consistency you like. At the last 2 minutes of beating, switch to high speed then chill the frosting if not using immediately.
Assembly
To fill cupcakes, you can use a cupcake corer or simply use a thin sharp knife to carve a small hole in the middle of your cupcakes about an inch in diameter.
Fill with a dollop of dulce de leche. Top with frosting and decorate as desired.
Video
Notes
If your buttercream curdles, just keep beating and your frosting will eventually come together.
Make sure your cupcakes have completely cooled before frosting.
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