Brew the coffee and pour it into a glass. Put the glass with coffee into a small bowl and fill the bowl with hot water just up to coffee level. This trick will keep the coffee hot while you'll be preparing the topping.
To prepare egg cream, whisk together egg yolks, heavy cream, erythritol and vanilla extract until thick and foamy.
Take the glass out of the bowl with hot water. Pour the cream on top of the coffee and enjoy.