In a small sauce pan, heat the sweetened condensed milk over medium heat. Add the chocolate; stir with a wooden spoon until the chocolate is melted. Add butter and vanilla; mix until creamy and smooth.
Remove from heat and stir in most of your “add ins”, reserving a small handful to decorate the top of the fudge.
Line an 8x8-in. pan with parchment or plastic wrap and pour in chocolate mixture. Decorate with your add-ins and refrigerate for at least 2 hours.
To cut, remove the entire fudge from the pan and place on cutting board; cut straight through with your knife. Clean your knife between cuts to make cutting easier.