Preheat your oven to 350°F (175°C). Grease and line three 8-inch cake pans with parchment paper, then dust each with cocoa powder.

In a large mixing bowl, whisk together flour, cocoa powder, baking soda, baking powder, and salt. Set aside.

Using an electric mixer, beat the butter and sugar until light and fluffy, about 3-5 minutes. Add Nutella, eggs, and vanilla extract; mix well.

Alternately add the dry ingredients and buttermilk to the creamed mixture, mixing until just combined.

Divide the batter evenly among prepared pans and bake for 25-30 minutes. Cool in pans for about 10 minutes, then transfer to wire racks.

In a heatproof bowl over simmering water, whisk the egg whites, sugar, and salt until dissolved. Beat on high until glossy peaks form, then gradually add butter and Nutella until smooth.

Stack the cooled cake layers with frosting in between and coat the outside with frosting. Chill for at least 30 minutes before serving.

Optionally garnish with chocolate shavings before serving.