Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt.

In a separate bowl, mix hot water and espresso powder until dissolved. In another bowl, whisk eggs, milk, yogurt, and vanilla until smooth.

Gradually add the dry ingredients into the wet mixture, alternating with the hot espresso water until just combined.

Divide the batter evenly between the prepared pans and bake for 30-35 minutes or until a toothpick comes out clean.

Prepare the mousse by dissolving cocoa and espresso in hot water, then melt the chocolate chips. Whip heavy cream with sugar until stiff peaks and fold in cocoa mixture and Nutella.

Once the cakes are cool, slice each into two layers, layer them with mousse, reserving some for decoration.

Refrigerate the assembled cake for at least 2 hours.

For ganache, heat cream until simmering, pour over chocolate chips and stir until smooth. Add butter if desired.

Pour ganache over the cake, allowing it to drip down the sides for coverage.