Melt the chocolate and butter in a water bath. Stir and let cool to room temperature (otherwise marshmallow will melt). Put marshmallow into the chocolate mixture, mix well. Marshmallow must be covered with chocolate completely. 

Lay the foil or parchment, put on it marshmallow and wrap as sausage, tie the ends and place in refrigerator for 4 hours or overnight. take out only before serving.