Mix the cocoa and sugar together well until they are of a single consistency.
In a double boiler melt the butter and peanut butter together. Pour the melted mixture into the bowl with the sugar/cocoa mixture. Mix well (but not with a mixer, but rather with your hands or a wooden spoon).
Grease a 9-inch square pan. Place the fudge mixture into the pan and pat down until evenly distributed. Cut the fudge as desired (this is important to do before you go to the next step because it is not easy to cut after this point). Refrigerate the fudge until it is hard.
