In a small saucepan, melt the butter over medium heat. Add the Guinness beer and bring to a simmer. Let it simmer for 5 minutes, reducing slightly.

Remove from heat and add the chopped dark chocolate. Stir until completely melted and smooth. Let the mixture cool to room temperature.

In a separate mixing bowl, whip the chilled heavy cream, powdered sugar and vanilla until soft peaks form.

Gently fold the cooled Guinness-chocolate mixture into the whipped cream until fully combined.

Spoon the mousse into small glasses, leaving a gap at the top for cream, and refrigerate for at least 2 hours before serving.

To serve: Whip the cream, powdered sugar and vanilla until soft peaks form. Spread onto the glasses of mousse to look like the foam on Guinness.

Serve chilled.