Using a hand mixer with a whisk, beat the egg yolks with sugar until fluffy.
Add lemon or orange juice and beat for another minute or so.
Add half the portion of the mascarpone and continue beating the mixture until well combined and creamy.
Add the remaining portion of the mascarpone cheese and continue beating until the mixture is smooth and creamy.
Using a spoon, slowly add the cocoa powder as you continue mixing the creamy mixture.
Beat the mousse for another 3-5 minutes, until you obtain a stiff and creamy consistency.