Chop the chocolate and melt it in a metal bowl over hot water, stirring occasionally. (See Note 2)
Beat the egg yolks and sugar until the mixture is fluffy and light. Save egg whites for other use.
Using an electric mixer on slow speed, beat the melted chocolate into the egg-sugar mixture a little at a time until just combined, working quickly so the chocolate does not harden. Beat the mixture on medium speed for 2 minutes. Reduce the speed to slow and blend in the orange extract. Set the mixture aside.
Using a chilled bowl and beaters, whip the cream until it forms very stiff peaks, incorporating the vanilla as you beat. Using a whisk, fold half of the whipped cream into the chocolate mixture, blending completely to lighten it. Using a chopping motion with a spatula, fold in the remaining whipped cream.
Refrigerate the mousse at least 8 hours or overnight (don't skip this step or the mousse won't have the proper consistency) and spoon or pipe into dessert bowls to serve. Feel free to top with shaved chocolate and serve with strawberries (optional).
The mousse will keep refrigerated for 2 to 3 days. This batch yields 10 servings.
Video
Notes
In French restaurants, mousse au chocolat is typically made with either bittersweet or dark chocolate. Feel free to use semisweet if you prefer. For the best results, chop up bars of chocolate rather than using chocolate chips or chunks.
Adapted from Piret Munger's Chocolate Mousse recipe.