Preheat your oven to 350°F (175°C). This ensures that the cupcakes bake evenly.
Line a cupcake pan with cupcake liners. This makes for easy removal and adds a lovely touch to your cupcakes.
In a large mixing bowl, use an electric mixer to cream together the 5 1/2 tablespoons of unsalted butter and the granulated sugar until light and fluffy. This should take about 2-3 minutes.
Add in the large egg, the egg yolk, and the vanilla extract. Mix until well combined.
In another bowl, whisk together the all-purpose flour, baking powder, and salt.
Gradually add the dry ingredients to the butter mixture, alternating with the sour cream and milk. Start and end with the dry ingredients, mixing until just combined.
Using a scoop or spoon, fill each cupcake liner about two-thirds full with batter. This allows room for the cupcakes to rise without overflowing.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the cupcakes from the oven and let them cool in the pan for about 5 minutes. Then transfer them to a cooling rack to cool completely.
In a clean mixing bowl, beat the 8 tablespoons of unsalted butter until creamy. Gradually add the powdered sugar, cocoa powder, and heavy cream. Mix until well combined, then add the Nutella and mix until smooth. If the frosting is too thin, add more powdered sugar until you reach your desired consistency.
Once the cupcakes are completely cooled, frost them with the Nutella buttercream. You can use a piping bag for a decorative touch or simply spread it on with a knife.
If desired, top your frosted cupcakes with colored sprinkles for a fun and festive finish.