In a medium saucepan, stir together the chocolate chips, heavy cream, butter, corn syrup, and cocoa powder. Heat the mixture over medium heat, stirring constantly, until the chocolate is melted and the mixture is smooth, 3-5 minutes. Don't let the mixture overheat or scorch! Add additional milk or heavy cream, if needed, for a thinner consistency.

Remove the pan from the heat and stir in the vanilla. Let cool slightly before serving. Store in a covered container in the fridge for up to 2 weeks. Reheat in the microwave in 30 second intervals, stirring in between.