In saucepan, combine water, sugar, and cocoa. Simmer mixture over medium heat 2 minutes, stirring frequently. Reduce heat and add milk in a slow stream, beating vigorously, until well combined. Add 2 Tbsp chocolate chips. Continue to heat until chocolate is completely melted, but do not boil. Divide mixture among 4 heatproof glasses and then fill with hot coffee. Garnish each drink with a dollop of whipped topping, and sprinkle with chocolate chips.