1Preheat oven to 350 degrees Fahrenheit.
2Melt 125 grams of dark chocolate with 60 milliliters of water and set aside.
3In a medium-sized bowl, mix together the flour, unsweetened cocoa powder, baking powder, baking soda, and a pinch of salt.
4In another bowl, beat 125 grams of softened butter, the granulated sugar, and dark brown sugar until the mixture is pale.
5Add eggs, one at a time, while continuing to whisk.
6Alternate adding the flour mixture and the buttermilk to the above mixture, and mix.
7Finally, add the melted chocolate.
8Divide the batter into two buttered and flour cake pans, then bake for approximately 35 minutes.
9In a double boiler, melt 125 grams of dark chocolate then set aside to cool completely.
10In a bowl, whisk 227 grams of softened butter with powdered sugar on low speed for about 1 minute.
11Add vanilla extract and beat again on low speed until everything is well blended.
12Add the melted chocolate to the butter mixture and beat at medium speed until everything is smooth.
13Add Nutella and milk. Beat on medium-high speed for another minute.
14Once the cakes cool down, assemble by layer the cake and the frosting.
15Make sure to frost your cake in several increments, placing it in the refrigerator between each layer.