Melt the chocolate using a double boiler or in the microwave. Beat the egg yolks with half of the sugar. Heat the milk with the remaining sugar. Gradually pour the hot milk over the yolks while stirring constantly. Return the mixture to low heat and cook until it thickens. Remove from heat, add the melted chocolate, and keep stirring regularly until the mixture cools.

Whip the heavy cream until stiff peaks form, then gently fold it into the cooled chocolate mixture.

Melt the butter and brush it inside the ramekins. Cut strips of parchment paper and line the inside edges of the ramekins, letting the paper extend a few inches above the rim.

Fill the ramekins with the mixture and freeze overnight. Remove from the freezer 10 minutes before serving. Take off the parchment paper and dust with unsweetened cocoa powder.