Position a rack in the center of the oven. Pre-heat to 375°F. Line two standard muffin tins with 12 paper liners.
Boil ¾ cup water (stove or microwave), set aside.
In a large mixing bowl, whisk together the sugar, flour, cocoa powder, baking powder, baking soda, and salt until no visible lumps.
Add in egg, sour cream, oil, and vanilla. Mix until just smooth and combined.
Pour in the boiling water and slowly mix, starting from the center and working your way outward. The batter will be quite liquidy like brownies, but this is normal.
Divide the batter evenly among the lined cups. Fill each one about halfway with ¼ cup batter.
Bake 15 to 16 minutes, until a cake tester or a wooden toothpick inserted into the center comes out clean.
Transfer the tins to wire racks for about 5 minutes until the tins are cool to the touch. Remove and transfer them to the racks. Let the cupcakes cool completely before frosting them and decorating. The cupcakes will keep in an airtight container at room temperature for up to 2 days.
Warm the milk until just boiling and pour over chocolate, stir, then add nuts. You can microwave the cream and chocolate mixture in a microwave-safe bowl for a few seconds if needed to melt the chocolate further. Cool in refrigerator and then transfer to a piping bag. Cut off the tip of the bag right before filling.
In a stand mixer whip 12 ounces butter on medium-high speed until light and fluffy, 2 minutes.
Gradually add 6 cups of sugar 1 cup at a time (low speed, then medium-high to smooth out). Add milk, mix on medium-high until combined.
Add 1 cup Nutella, mix on medium-high until smooth. Transfer to a piping bag with a large star tip.
Once the cupcakes are cooled, use a large melon baller or spoon to remove 1 teaspoon of cake from the center. Pipe in 1 teaspoon chocolate ganache and hazelnut filling.
Pipe the nutella buttercream in a swirl pattern on top of the filled cupcake. Decorate with desired toppings.