Melt the chocolate and butter stirring frequently in the top of a double boiler over simmering water. Add the cocoa and whisk until dissolved.
Using a large spoon, gradually stir in the sugar. The mixture will have the consistency of wet sand. Cook, stirring occasionally over simmering water for about 20 minutes. Check the water level in the double boiler if necessary, replenish if needed.
Gradually stir in the milk and cream and keep stirring until completely blended. Use whisk to get any lumps out. Continue cooking over low heat for about 1 hour. The fudge is ready when completely smooth and all of the sugar is dissolved. Store any leftovers in refrigerator.