1. For the filling, add the water, sugar, and butter to a small saucepan and heat up until the butter has melted. Add the ground hazelnuts and stir until you get a smooth paste. Remove from heat and let cool for at least 30 minutes. The filling can also be prepared a day ahead and stored in the fridge.

2. Preheat the oven to 350°F (180°C). Line a muffin tin with 12 paper liners and set aside. Add the butter and chopped chocolate to a microwave-safe bowl and melt slowly in the microwave. You can do this over a pot of simmering water as well. Mix everything to get a smooth chocolate sauce and set aside to cool slightly.

3. Add the eggs, sugar (or xylitol), and vanilla extract to a large bowl and mix until light and fluffy. Add the cooled chocolate sauce and stir to combine. Mix the flour with baking powder, and salt, add to the bowl, and mix in. Add the chopped hazelnuts and fold in. Fill the paper liners halfway with the batter. Use a teaspoon to place small portions of the filling on top of the batter in the paper cups and then cover with the remaining batter. The paper liners should be about 2/3 full at the end. Bake the cupcakes for 20-25 minutes or until a toothpick inserted into a cupcake comes out clean. Take out of the oven and let cool down completely on a wire rack.

4. For the buttercream, mix the softened butter and Nutella together in a large bowl until light and fluffy. Add the confectioners’ sugar, vanilla extract, and salt and mix for 6-5 minutes until fluffy. If the buttercream seems too firm, add some milk and mix again. Fill the buttercream into a piping bag with a star tip and pipe it onto the cupcakes. Decorate with a few chopped hazelnuts (optional).