Preheat the oven to 350°F and generously spray two 8- or 9-inch round cake pans (or you can do this with one cake pan, baking one at a time) with cooking spray. Using a stand mixer fitted with the paddle attachment (or a large mixing bowl with a hand mixer), combine the butter, sugars, eggs and vanilla and beat on medium speed until creamy and free of lumps. Add the milk and Nutella and mix until just combined.
In a separate bowl, combine the flour, cocoa powder, baking powder, baking soda and salt. Mix well. Gradually add to the butter mixture and beat on medium speed until just combined. In a small bowl, stir the coffee granules into the boiling water until dissolved. Add to the mixing bowl and mix until creamy (batter will be thin). Mix on medium speed for 2 full minutes.
Divide the batter between the prepared cake pans and bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let cool in pan for 5 minutes, remove from pan and let cool completely on a wire rack. Using a serrated bread knife, cut the rounded tops off each cooled cake.