Preheat your oven to 350°F. Line two 8-inch round baking pans with parchment paper.

Combine chopped chocolate with boiling water. Let stand for a minute before whisking until melted and smooth. Set aside to cool.

Cream butter with brown sugar and vanilla extract until pale and fluffy, 3-5 minutes. Beat in eggs, one at a time. After the last egg is incorporated, beat on high speed for a minute until the mixture is smooth, creamy and pale.

In a separate bowl, sift together flour, cocoa, baking powder, baking soda and salt. Add ground hazelnuts and whisk to combine. Add flour mixture to the butter mixture alternately with the cooled chocolate/water mixture until combined.

Divide batter evenly between pans and bake 23-27 minutes, or until a skewer inserted into the center comes out clean.

For the filling, melt butter and milk in a small saucepan over medium-low heat and bring just to a simmer. Add both types of chopped chocolate and let stand before whisking until smooth and glossy. Whisk in cocoa and salt. Whisk in icing sugar and beat constantly until thickened like fudge. Spread filling between cooled cake layers and dust with cocoa powder.