In a small heatproof bowl, combine the chocolate and butter. Set the bowl over a pot of barely simmering water. Stir constantly until chocolate is melted and smooth. Remove from heat but leave the pot of simmering water on the burner.
In a medium heatproof bowl, whisk the egg yolks and sugar together until well combined. Set the bowl over the pot of barely simmering water and whisk for 5-10 minutes, until the sugar has dissolved and the mixture turns pale and becomes slightly stiff. Remove from heat.
Stir in the vanilla followed by the chocolate mixture. Stir for a few more minutes to allow the mixture to cool then set aside until it cools to room temperature.
In a large bowl, whip the cream to soft peaks. Whisk ¼ of the whipped cream into the cooled chocolate mixture. Use a silicone spatula to gently fold in the remaining whipped cream until fully mixed in.
Divide the mousse among four 6-ounce ramekins and refrigerate until set, about 1 hour.
Garnish with whipped cream, chocolate shavings, or strawberries if desired.
Notes
Bittersweet chocolate can be used in place of semisweet chocolate.
1.5 ounces of coffee or liqueur can be used in place of vanilla extract.