Whip 1 cup of the heavy cream in the bowl of a stand mixer fitted with a whisk attachment until medium stiff peaks form. Do not over whip. Chill in the fridge until needed.
Melt the chocolate in a heat proof bowl set over a pot of barely simmering water. Stir until melted and smooth. Remove from heat. Alternatively, you may melt the chocolate in the microwave on 50% power in 1 minute intervals, stirring between each.
In another heat proof bowl, combine the egg, egg yolks, and sugar. Use a hand held mixer or wire whisk to beat the eggs and sugar until pale yellow, about 2 minutes. Place the bowl over a pot of barely simmering water and continue beating until mixture is warm to the touch, about 5 minutes. Scrape down the sides of the bowl as necessary. Remove the bowl from the heat and continue beating until the mixture comes off of the whisk in thick ribbons and had cooled to room temperature, about another 3 minutes.
Slowly begin folding the melted chocolate into the egg mixture until it is all combined and is uniform in color and texture. Fold ⅓ of the whipped cream into the chocolate mixture to lighten it. Fold in the remaining whipped cream, being careful not to over beat it and cause it to loose it's volume.
Transfer to serving cups and chill for at least 2 hours.
When ready to serve, whip the remaining ½ cup whipping cream in the bowl of a stand mixer fitted with a whisk attachment. Add confectioners sugar and vanilla. Spoon over chilled mouse and sprinkle with cinnamon, if desired.