Make the Base. Add butter to a medium-sized saucepan over medium heat. Once butter has melted, add sugars, milk, and cocoa powder. Bring to a boil and let boil for 1 minute, stirring occasionally. Remove from the heat and stir in peanut butter, vanilla, and salt.
4 Tbsp. butter, ¾ cup granulated sugar, ½ cup brown sugar, ½ cup milk, ¼ cup cocoa powder, ⅔ cup peanut butter, 2 tsp. vanilla extract, ¼ tsp. salt