Mocha Quatro Leches Cupcakes

For the filling:
Preheat the oven to 425°F. Pour the condensed milk into a small, shallow baking dish and cover tightly with aluminum foil. Place the baking dish in a larger roasting pan. Fill the roasting pan with enough water to come halfway up the sides of the baking dish. Roast for 1 hour and 15 minutes, stirring once while cooking, until the milk turns a light golden color. Remove the baking dish from the water and transfer the dulce de leche to a small bowl. Let it cool completely.

For the cupcakes:
Decrease the oven temperature to 350°F. Line 16 muffin cups with paper liners.

Whisk together the flour, baking powder, and salt in a small bowl.

Beat the butter and sugar in the bowl of an electric mixer on medium speed for 2 to 3 minutes, until light and fluffy. Whisk together the sour cream, milk, and vanilla extract in a small bowl to combine. With the mixer on low speed, add the flour and milk mixtures alternately to the batter, beginning and ending with the flour mixture and blending until just incorporated.

Beat the egg whites and cream of tartar in a separate bowl until stiff peaks form. Fold one-fourth of the egg whites into the batter. Gently fold in the remaining egg whites, mixing just until no white streaks remain.

Using an ice cream scoop, scoop the batter into the muffin cups. Bake for 15 to 20 minutes, until a cake tester comes out clean. Remove from the oven and let them cool for 10 minutes. Transfer the cupcakes to a wire rack and let them cool completely.

For the sauce:
Whisk together the condensed milk, evaporated milk, and heavy cream in a small bowl to combine.

For the frosting:
Beat together the butter, powdered sugar, and cocoa powder in the bowl of an electric mixer on medium speed for about 2 minutes, until light and fluffy. Add the coffee, vanilla extract, and salt and mix until smooth.

For the assembly:
Spoon the dulce de leche into a pastry bag fitted with a large round tip. Gently insert the tip of the pastry bag into the top of one cupcake and fill with some dulce de leche. Using a toothpick, prick the cupcake all over and brush with some of the milk sauce. Spread a generous amount of frosting on top of the cupcake. Repeat with the remaining cupcakes, dulce de leche filling, tres leches sauce and frosting.