Pour brewed coffee, half and half (milk or cream), sweetener, and Nutella into a small sauce pan set over medium low heat.
Stir until the Nutella melts and all ingredients combine.
If you had your coffee in the fridge, it may take 5 to 10 minutes. At room temperature, it will take about 5 to heat through. If you just brewed fresh, hot coffee, you just need to stir until combined.
Yes, you can have it plain, but it is obviously better when topped with whipped cream and chocolate sauce. You could also use chocolate shavings if you like.
Notes
Adjust sugar and cream to your desired amount. Fresh, home brewed coffee tastes best, but you can use coffee you saved or instant coffee if you prefer.