Preheat your oven to 350°F (175°C).
Melt the dark chocolate and butter together in a double boiler.
Remove the mixture from heat and let it cool slightly. Stir in the egg yolks one at a time.
In a separate bowl, whip the heavy cream to soft peaks.
In another bowl, whip the egg whites with sugar until stiff peaks form.
Carefully fold the whipped cream into the chocolate mixture.
Next, fold in the whipped egg whites until fully combined, being careful not to deflate.
Divide the mousse into three bowls for layering.
Layer the mousse in a prepared cake pan, starting with the dark chocolate layer.
Repeat the process with the milk and then the white chocolate layers.
Chill the cake in the refrigerator for at least 4 hours or overnight.
Once set, gently remove the mousse cake from the pan and serve with chocolate shavings or whipped cream, if desired.
