Preheat oven to 180° C. Place cupcake liners in a mold for 12 cupcakes.
In a large heatproof bowl, place the cocoa powder with the instant coffee and add the boiling water. Mix with a balloon whisk and let stand until cool.
In a medium bowl, sift the flour with the salt, baking powder and baking soda.
To the bowl with the chocolate mixture, add the melted butter with the two types of sugar, eggs, nutella, sour cream and vanilla extract. Beat with an electric mixer or balloon whisk for 2 minutes until smooth and homogeneous.
Add the flour mixture in three additions, beating for 30 seconds after each addition, until the mixture is lump-free and smooth.
Pour the mixture into the muffin cups filling each one 3/4 full and bake at 180° C for 20 minutes until lightly browned and cooked.
Once ready, remove the cupcakes from the oven and let them cool completely before decorating.
For the frosting:
Place the chocolate with the whipping cream in a heatproof bowl. Melt in the microwave for 30 second intervals, that is, put it in the microwave for 30 seconds and take it out after this time to mix a little with a fork. Repeat the process until the chocolate is completely melted (this prevents it from burning).
Once the chocolate is melted, add the nutella to the same bowl and mix with a balloon whisk until you get a very smooth and liquid frosting. Cover the bowl with plastic wrap and refrigerate the frosting for 30 minutes until it is thick enough to decorate the cupcakes.
Once the chocolate is melted, add the nutella to the same bowl and mix with a balloon whisk until the frosting is very smooth and liquid. Cover the bowl with plastic wrap and refrigerate the frosting for 30 minutes until it is thick enough to decorate the cupcakes.
Once the frosting has a creamy but firm consistency, place it in a piping bag with a star or round doughnut (or directly with a spoon) and decorate the cupcakes in the way you prefer.
To decorate, add chopped hazelnuts on top of each cupcake and that's it.