To make the cake, preheat the oven to 350°F (175°C). Grease two 9-inch round cake pans.
In a medium bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder and salt. Set aside.
In a small bowl, mix together water and instant espresso coffee. Set aside.
In a mixing bowl with paddle attachment, mix together eggs, milk, yogurt and pure vanilla extract until well combined. Add half of the flour mixture for a minute, then add the remaining mixture and mix until just combined. Stir in the espresso water.
Pour the batter into two cake pans. Bake for about 30-35 minutes, or until inserted toothpick comes out clean. Cool on wire racks for 10 minutes. Then remove the cakes from the pan and cool completely.
Meanwhile, let’s prepare the chocolate mousse filling. In a small bowl, dissolve cocoa powder and instant espresso powder in a hot water.
In a double boiler, melt chocolate chips until nice and smooth. Remove from heat and cool slightly.
In a mixing bowl with whisk attachment, whip the heavy cream with the sugar until hard peaks form.
Stir in the cocoa mixture and Nutella into the melted chocolate. Add half of the whipped cream and mix everything together with folding motion. Fold in the remaining whipped cream and mix well until no white streaks remain. Refrigerate until the cake is cooled completely.
To assemble the cake, slice the cakes horizontally into two equal layers, using a long serrated knife. If your cakes got a small hump as pictured, just slice it off.
Put the first cake layer into a springform pan and spread about a little less than 1/3 of the mousse filling. Then put the next layer of the cake and spread the mousse and so on until the cake is built. Make sure to leave a bit of mousse to cover the top of the cake. A very thin layer of mousse on top is all you need. Refrigerate the cake for at least 2 hours or overnight.
Once the cake is cooled, make the chocolate ganache. In a small saucepan, bring heavy cream to a simmer. Do not boil. Pour hot cream over chocolate chips, and let it sit for good 2 minutes. Then mix well until smooth for couple minutes. Stir in butter to add some shine.
Remove the springform sides. Pour the ganache over the cake while the ganache is still runny, if you want smooth glaze. Or, you can let it cool and thicken a little bit to spread as a frosting.