Toast hazelnuts in a dry skillet over medium heat until fragrant, about 5-7 minutes. Blend in a food processor until smooth to make hazelnut butter.

Preheat oven to 160°C (325°F) and prepare two cake pans. Whisk dry ingredients together in a bowl. In another bowl, mix wet ingredients and combine with dry ingredients, adding hot water. Divide into pans and bake for 23-25 minutes.

Repeat the above step for hazelnut cake layers.

Heat 200g of heavy whipping cream until bubbling, then pour over chocolate and hazelnut butter in separate bowls. Stir until smooth and let cool.

Combine 225g butter, flour, sugar, salt, and milk over medium heat, whisking until thick. Cool and beat with remaining butter and vanilla until fluffy.

Level cooled cake layers, stack them with ganache in between, apply a crumb coat of buttercream, and chill for 30 minutes.

Frost the chilled cake with remaining buttercream and decorate as desired.