First melt the chocolate in a bowl over a pan of simmering water. Once it's melted take it off the heat to save it burning. You could also melt the chocolate in a microwave.
Next melt the butter and add it to the chocolate along with the egg yolks. Stir until you've got a smooth, silky mixture.
In another bowl whisk the egg whites (use an electric mix if you've got one) until they form soft peaks. Add the sugar a little at a time and continue to whisk until the mixture is shiny and the peaks are stiff.
Fold the egg whites into the chocolate mix along with the Baileys. Then divide your mousse between four tumblers and pop it in the fridge for a couple of hours.
When you're ready to serve, whip up the whipping cream until it forms stiff peaks.
Add a good spoonful of whipped cream to the top of each mousse and decorate with crumbled or flaked chocolate.
Short on time? Chill your tumblers first then divide the mixture between them before popping them into the fridge for about 30 minutes.
We recommend you serve your mousse in a chunky glass tumbler for a cosmopolitan feel, but you could also serve it in a chic cocktail glass, earthy ceramic pot or traditional trifle bowl.
Feeling fruity? Decorate your mousse with a few raspberries or black cherries.