Use a microwave or a double boiler to melt the chocolate. Stir every 20 seconds so it does not burn the base. Let cool.
Dissolve a scoop of collagen in 1/4 cup coffee.
Beat the egg yolk until pale, add to the melted chocolate and stir well.
Pour collagen coffee into the egg-chocolate mixture and stir to combine. Whip ¾ cup of heavy cream to stiff peaks. Fold it to the collagen-coffee-chocolate mixture. Add liquid stevia in and stir to mix.
Divide the mixture into 2 glasses (or 4 small glasses for 4 servings) and transfer to a fridge for at least 30 minutes.
Whip ¼ cup of the remaining cream. Garnish the mousse with whipped cream and cocoa powder on top and serve.