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- Butter a glass loaf pan 8.5 x 5.5. Line the dish with plastic wrap, allowing it to drape down over the side. Take care to make it as smooth as possible.
- Add the chocolate chips to a medium bowl set over a pan of gently simmering water. Stir the chips until they are completely melted. Remove the chocolate and keep warm. Turn down the heat under the water and keep it warm. You’ll need it again in a few steps.
- In another medium bowl, beat 1/2 cup of butter until light and fluffy. Add 1/4 cup of sugar and the sifted cocoa powder. Blend until smooth.
- In a large metal bowl, whisk the eggs yolks, 1/4 cup of water, 1/4 cup butter and 1/4 cup of sugar. Set the bowl over the saucepan of gently simmering water (don’t let it touch the water) and whisk the egg mixture constantly until a candy thermometer reads 160 degrees, about 5-6 minutes. Remove the egg mixture from over the water and beat with an electric mixer until it is thick and cool, about 5 minutes. Beat in the cocoa mixture and fold in the warm chocolate and vanilla.
- In a separate clean bowl beat the whipping cream until soft peaks form. Gently fold the cream into the chocolate mixture. Do no over-beat. Spoon the mousse into the prepared pan and cover and chill until firm (about 4 hours).