STEP 1 Com­bine choco­late and but­ter in a heat­proof bowl and set over a saucepan of barely simmering wa­ter un­til just melted, stir­ring with a metal spoon to com­bine. Set aside.
STEP 2 Put yolks, mus­co­v­ado sugar and muscat in a large heat­proof bowl and set over a saucepan of simmering wa­ter. Whisk con­stantly for 5 min­utes, un­til thick­ened, then gen­tly fold in choco­late mix­ture us­ing a spat­ula. STEP 3 Spoon mousse into 4 warm coupe bowls, top with hazel­nuts, waf­fle cone pieces, rasp­ber­ries and mini marsh­mal­lows. Dust with choco­late pow­der and dol­lop with cream. Serve.