In a saucepan, combine the whipping cream, dark chocolate,and butter. Warm gently over low to medium heat, stirring constantly until everything is melted and smooth.
Do not let it boil; just heat until fully combined and glossy.
Remove from heat and whisk in the sifted cocoa powder, vanilla extract, and sifted powdered sugar until smooth and well blended.
Strain the frosting into a bowl for an extra silky texture, then press plastic wrap directly onto the surface.
Let it cool in the fridge for 1 hour 30 min (don't let it set too firm, it should have thick spreadable consistency), or ideally overnight in a cool room temperature, until thick and spreadable.