Put butter, cocoa and sugar in a saucepan with the milk. Cook over medium heat until the mixture boils. Let boil 1 minute and remove from heat. Add vanilla. Cool.
Stir in confectionery sugar. Add a little milk if necessary.
Recipe by Recipes Food and Cooking at https://recipesfoodandcooking.com/2015/10/07/lindt-brownie-bites/