1/4 cup cocoa powder
2 cups sugar
1/2 cup milk
1 stick salted butter
1 teaspoon vanilla extract

Mix cocoa, sugar and milk in a pot. Add butter and cook over high heat while stirring for about 8 minutes or until a candy thermometer reaches 250 F. Place bottom of pot in a bowl or sink full of cold water and continue stirring until smooth. Stir in vanilla extract. Pour onto buttered surface. A marble cutting board works great. Shape with a buttered knife. Let cool and slice with a buttered knife. 

Recipe note: This fudge has a firmer texture than some recipes.