Melt the chocolate and butter, stirring frequently, in the top of a double boiler over simmering water. When chocolate and butter are melted, add the cocoa powder and whisk until dissolved.
Using a slotted spoon, gradually stir in the sugar. The mixture should be the consistency of wet sand. Cook, stirring occasionally, over simmering water for 20 minutes. Check the water level in the double boiler occasionally and replenish if necessary.
Gradually stir in the milk and cream and keep stirring until completely blended. Continue cooking, stirring and checking the water occasionally, for 1 hour. The fudge is ready when it is completely smooth and all the sugar is dissolved. Store leftovers in fridge; it lasts a "long time" according to the book.