Chocolate Mousse

This recipe is from James Peterson's BAKING. 

6 oz chocolate (he recommends bittersweet, I do half dark half milk)
1/4 c plus 2 T unsalted butter, cubed or sliced
4 egg yolks
3 T sugar
1 t vanilla extract
1 c heavy cream and possibly more if needed to unseize chocolate

Chop the chocolate and combine it with butter in a double boiler (heatproof bowl over a saucepan of barely simmering water) and stir until melted and smooth.  Take off heat, but keep water simmering.

Combine egg yolks and sugar in another heatproof bowl and set over saucepan of simmering water for 5 minutes, whisking constantly, until pale and slightly stiff.  (It will thicken up.) Remove from heat.

Stir the chocolate mixture into the egg mixture and add the vanilla and stir until well mixed.  If it becomes grainy at this point (it should be smooth and shiny) slowly add 1/4 c warmed cream, a little at a time, until it becomes shiny and smooth.  Let cool to room temperature.

Whip cream to soft peaks.  Stir about 1/4 of whipped cream into the chocolate mixture to lighten.  Then FOLD in the remaining whipped cream with a rubber spatula.

If you are using as a layer in a cake use it right away, otherwise chill for at least 1 hour.  If you are serving the mousse alone pour into individual glasses immediately and cover with plastic wrap (remember to press it against the surface) to keep a crust from forming.

This recipe makes 3 cups.  The first time I made it the chocolate mixture seized, the second time it didn't.  This is one of our favorite desserts.