In a medium or large pot, add sugar, cocoa powder, butter, and milk.
Cook on low to medium heat while stirring constantly. Once the mixture comes to a boil use a candy thermometer and watch the temperature. As soon as it approaches a soft ball stage remove it from the heat.
Soft ball stage is 235-240 degrees F. So once the mixture gets to about 234 degrees I will remove it from the heat because overcooking this fudge will ruin it.
Once you remove the fudge mixture from the heat then quickly add in the peanut butter and continue stirring until the mixture is completely smooth.
Once the fudge mixture is smooth pour it into your buttered dish and let it cool. It should be firm enough to cut and serve within 10 minutes.