In a large pot with boiling water, place a can of sweetened condensed milk sideways (not standing up) – make sure there are no dents on the can. Simmer on medium heat for 2:20 hours. When time is up, carefully remove the can and let it cool completely.
Preheat oven to 350ºF. In a large bowl, add flour, baking powder, baking soda and sea salt; whisk until well combined and set aside.
In a bowl of a stand mixer with the paddle attachment, cream brown sugar and butter on medium speed. Add eggs, one at the time, beating well between each addition. To this, add dulce de leche and buttermilk; process on low speed and slowly increase to medium-high for about 1 ½ minutes, stopping and cleaning the sides of the bowl.
Add the dry ingredients to the dulce de leche mixture and process on low speed until just blended, about 45 seconds. Add oil and pure vanilla extract and process for 30 seconds; the batter should have a nice smooth texture.
Insert cupcake liners in a muffin pan and using an ice cream scoop, fill up the cups with the batter about 2/3 full. Gently tap the pan on the counter to remove any trapped air bubbles before transferring to preheated oven; bake for 18 to 20 minutes or until a cake tester inserted in the center comes out with only a few crumbs. Let the cupcakes cool off in the pan for 5 minutes before transferring them on a wire rack to cool completely.
Meanwhile, in a bowl of a stand mixer with the paddle attachment, add cream cheese and butter; process until mixed. Add 2 tablespoons dulce de leche and process until fairly well combined. Add half of the confectioners’ sugar and on low speed, process until mixed then increase to medium-high. Repeat the same steps with the other half, cleaning the sides of the bowl. It’s optional but if wanted, add a tiny pinch fleur de sel and if so, process.
Spoon the buttercream into a piping bag with a large tip and squeeze out the mixture onto the cupcakes. Warm up 2 tablespoons dulce de leche in the microwave until just pourable and drizzle over the buttercream. If wanted, sprinkle on a few grains of fleur de sel and refrigerate for 1 hour. Makes 12 cupcakes.