Melt the dark chocolate in a heatproof bowl over a saucepan of simmering water (double boiler method), stirring until smooth and remove from heat.
Add Irish cream flavored coffee creamer to the melted chocolate, stir until fully combined and let it cool slightly.
In a small saucepan, whisk together the egg yolks, sugar, and 1/4 cup of heavy cream, cook over low heat, whisking constantly, until the mixture thickens slightly (about 3-5 minutes), do not let it boil.
Remove from heat and whisk the egg mixture into the melted chocolate until smooth.
In a large mixing bowl, whip the remaining heavy cream until soft peaks form.
Gently fold the whipped cream into the chocolate mixture in batches, being careful not to deflate the mousse, ensure it is well combined but still airy.
Spoon the mousse into serving glasses or ramekins, cover and refrigerate for at least 2 hours or until set.
Before serving, top with whipped cream and chocolate shavings if desired.