Remove from the oven and let it cool.
Dulce de Leche
Once the butter is all incorporated add the dulce de leche, melted and cooled down chocolate, and espresso powder to the buttercream.
When the cupcakes have cooled down, use a spoon to remove the center of each cupcake.
Place the dulce de leche in a piping bag and pipe in the center of each cupcake. Or simply spoon a small amount (about 2 tsp) of dulce de leche in the center of each cupcake.
Top the cupcakes with the piece of cake you removed from the middle.
Place the Dulce de Leche Mocha Swiss Meringue Buttercream in a piping bag fitted with the tip of choice.
Pipe on top of the cupcakes.
I topped my cupcakes with crispearls by Callebaut.
The cupcakes will last up to 5 days in the fridge, well covered.
Dulce de Leche: you can use homemade dulce de leche, or make it at home. If using store-bought, you will need 1 1/4 cup of dulce de leche. This is one of my favorite Dulce de Leches in the whole world: Havanna Argentina Dulce De Leche.
Crispearls: I used crispearls by Callebaut to top the cupcakes. You can find them in these links: Milk Chocolate Crispearls, White Chocolate Crispearls, Dark Chocolate Crispearls, Salted Caramel Crispearls.
Piping tip: This is the piping tip I used for these cupcakes: Wilton 1B Drop Flower Tip
Swiss Meringue Buttercream: I can't stress enough how important it is for the butter to be exactly at room temperature, not cold, and not too warm either. If the butter is too cold, it will form lumps in the buttercream, and if the butter is too warm, the buttercream will be soupy.
Meringue: the meringue needs to be at stiff peaks before you start adding the butter, or the buttercream won't firm up.
Troubleshooting the Swiss Meringue Buttercream: if the buttercream is too soupy and runny, it's either because the meringue wasn't stiff before adding the butter, or because the butter was too warm when you've added it. Place the bowl in the fridge for 5 to 10 minutes if the buttercream isn't firming up. And then whip it again for a few minutes and that will probably solve your issue. If the buttercream doesn't firm up, it's because the meringue was too soft when you've added the butter, and there's not much that can be done at this point.
Melted chocolate: make sure the chocolate has cooled down entirely before adding it to the frosting, or it will melt the butter.
Chocolate cupcakes: these cupcakes are SUPER soft and fluffy. I recommend placing them in the fridge for an hour or so before filling with the dulce de leche and decorating.