Grease an 8"x8" baking dish, set aside. In a medium sauce pan combine sugar, cocoa, butter, and milk. Heat over medium heat, stirring constantly until sugar is dissolved and the mixture comes to a boil. Once the temperature reaches 230-240 degrees F. Remove from the heat, you do not want to cook past that point.
Add in the peanut butter and stir until melted and smooth. Pour the fudge mixture into the prepared baking dish, top with crumbled Reese's cups and let chill until set. Slice and enjoy.