Melt the chocolate and butter in a double boiler. Set aside to cool a little.
Whip the cream with an electric mixer on high for about a minute. Turn the mixer to low while adding the sugar, then turn back to high and whip to stiff peaks.
Whisk the egg yolks in a pitcher and add the vanilla.
Next, temper the eggs by adding a little of the melted chocolate. Then pour the eggs back into the bowl of melted chocolate and stir to combine. Fold in three-quarters of the whipped cream (refrigerate the remaining whipped cream for serving). Spoon the mousse into serving glasses or bowls and refrigerate for at least 1 hour.
Serve the mousse with dollops of whipped cream and garnish with grated chocolate and gummy worms.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.