Preheat your oven to 350°F (175°C). Grease three 8-inch round cake pans and line the bottoms with parchment paper.

In a mixing bowl, whisk together the dry ingredients: flour, cocoa powder, baking soda, baking powder, and salt until well combined.

In another bowl, beat the butter and sugar until light and fluffy. Incorporate Nutella, eggs, and vanilla until smooth.

Gradually add the dry mixture to the creamed mixture, alternating with buttermilk until just combined.

Divide the batter between the cake pans and bake for 25-30 minutes or until a toothpick comes out clean.

Let the cakes cool in the pans for 10 minutes before inverting onto a wire rack to cool completely, about 1 hour.

Combine egg whites, sugar, and salt in a heatproof bowl over simmering water, whisking until the mixture reaches 150°F (65°C).

Remove from heat and whip on high speed until glossy with stiff peaks.

Gradually mix in butter and Nutella until the frosting is smooth and creamy.

Put one layer of the cake on a serving plate. Spread a layer of Swiss meringue frosting, repeat with the second layer, and top with the third layer.

Frost the top and sides of the cake, smoothing it out for a clean finish.

Refrigerate the cake for at least 30 minutes before serving. Decorate with chocolate shavings if desired.