Begin by melting the semi-sweet chocolate chips. You can do this in a microwave-safe bowl in 20-30 second intervals, stirring until smooth, or using a double boiler. Let it cool slightly.
In a mixing bowl, pour in the heavy whipping cream, Baileys Irish Cream, powdered sugar, and vanilla extract.
Using a whisk or electric mixer, whip the mixture on medium-high speed until soft peaks form. This should take about 1-2 minutes.
Once the cream is whipped, slowly fold in the melted chocolate using a rubber spatula. Be gentle to maintain the light and airy texture of the mousse.
Spoon the mousse into serving dishes. You can pipe it for a more elegant presentation.
If desired, garnish with chocolate shavings or a sprinkle of cocoa powder.
Serve immediately or refrigerate for up to 2 hours before serving for a firmer texture.