How to make chocolate mousse
Place a bowl over a pan of simmering water (the water shouldn't touch the bottom of the bowl) and gently melt the chocolate in the bowl. Remove from the heat once melted and let it cool slightly.
Separate the egg yolks from the egg whites. Beat the egg yolks and most of the sugar together until creamy and pale in colour (keep two teaspoons of sugar to one side for the egg whites).
When it has cooled slightly, whisk the chocolate into the egg yolk and sugar mixture.
Melt the butter in a pan over a low heat.
Whisk the melted butter into the chocolate mixture. If it gets too thick, add a couple of tablespoons of water.
In a clean bowl, whisk the egg whites and the remaining two teaspoons of sugar with an electric whisk until they're light and fluffy and hold a soft peak. Do not over-beat. The sugar will give them a gentle sheen.
Carefully fold the egg whites into the chocolate mixture using a metal spoon.
Spoon the chocolate mixture into small teacups or ramekins and refrigerate for about two hours.
Just before serving, top each marquise with a dollop of crème fraîche and two fresh cherries, then sprinkle with cocoa powder.