Iced Irish Coffee

For 1 serving: Combine 1 cup freshly brewed hot coffee, ¼ cup heavy cream, and 1½ tsp. sugar (or to taste) in a mixing glass. Stir to dissolve the sugar, and chill thoroughly. Spoon 1 Tbsp. Sweetened Whipped Cream into the bottom of a 14-oz. highball glass. Stir 2 oz. Irish whiskey into coffee mixture, then pour the mixture over the whipped cream. Fill the glass with ice, then top with the remaining 1 Tbsp. Sweetened Whipped Cream. Dust with freshly grated nutmeg or ground cinnamon. Drink immediately.  For 4 servings: Combine 4 cups freshly brewed hot coffee, 1 cup heavy cream, and 2 Tbsp sugar (or to taste) in a pitcher. Stir to dissolve the sugar, and chill thoroughly. Spoon 1 Tbsp. Sweetened Whipped Cream into each of four 14-oz. highball glasses. Stir 8 oz. (1 cup) Irish whiskey into coffee mixture, then divide the cocktail among glasses. Fill each with ice and top with another 1 Tbsp. Sweetened Whipped Cream. Dust with freshly grated nutmeg or ground cinnamon. Serve immediately. Do ahead: Coffee mixture can be chilled up to 8 hours ahead.