Chop the milk chocolate into small pieces, place in a large heatproof bowl and set aside.
Measure out the cream, and pour roughly half of it into a small saucepan. Heat over medium-high heat until it comes to a boil.
Pour the cream over the chocolate and allow it to sit for a few minutes to melt the chocolate.
Stir with a whisk until the chocolate is fully melted and the mixture is combined. Whisk in the remaining cream, and then the Baileys.
Cover the bowl, and chill the mixture until very well chilled (at least several hours, overnight is better). If you're in a hurry you can chill it faster by putting it in the freezer for an hour or so. Make sure you stir it regularly and be careful not to let it actually freeze.
Whip the mousse mixture until stiff peaks are just beginning to form. Be careful not to overwhip it or it will become grainy - it's better to under-whip than over-whip.
Spoon the mousse into your serving glasses or bowls. Serve immediately for a softer, creamier mousse or chill for a firmer, more set mousse.
The mousse will keep for 2-3 days in the fridge. Cover individual glasses or dishes with plastic wrap, or place them all into a large airtight container.
