To make the fudge sauce: Combine chocolate chips, 2 tablespoons butter, 3/4 cup sugar, water, and corn syrup in a small saucepan. Bring to a boil, stirring continuously. Simmer for 5 minutes, remove from heat and stir in 1/2 teaspoon vanilla. Let cool completely and store covered, in the fridge (bring to room temperature before using).