Classic Chocolate Eclairs

Cut tops off of cooled pastry shells with a sharp knife. Fill shells with pudding mixture and replace tops.
Make icing: Melt chocolate and butter in a medium saucepan over low heat. Stir in confectioners’ sugar and vanilla. Stir in hot water, 1 tablespoon at a time, until icing is smooth and has reached desired consistency. Remove from heat and cool slightly.
Drizzle chocolate icing over filled eclairs. Store in the refrigerator until serving.