Preheat the oven: Preheat your oven to 350°F (175°C) and line a cupcake tin with liners.
Mix dry ingredients: In a bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.
Prepare the wet ingredients: In another bowl, whisk melted butter and sugar until they combine. Add the eggs, vanilla extract, and buttermilk—mixing well.
Combine: Slowly mix the dry ingredients into the wet ingredients, then add hot coffee or water to create a smooth batter.
Fill the tin: Fill the cupcake liners about 2/3 full with batter. Bake for 18-20 minutes until a toothpick inserted into the center comes out clean.
Cool down: Let the cupcakes cool completely before frosting.
Make Nutella buttercream: Beat the softened butter in a bowl until creamy. Add in powdered sugar, cocoa powder, and Nutella; mix until fluffy. Add vanilla and heavy cream, whipping until smooth.
Core and fill: Core the center of each cooled cupcake and fill with Nutella using a piping bag.
Frost: Pipe Nutella buttercream on top using a star tip and garnish with chocolate bars.
Serve and enjoy: These rich, chocolatey Nutella Bueno Cupcakes are ready to be devoured!