In a double boiler, once the water is simmering lightly add mixture into the top bowl or top boiler pot.
Stir until the erythritol is fully melted, takes approx 5 minutes. Test yolk mixture with fingers to make sure there is no grittiness. Keep stirring until smooth. Do not boil until the eggs will begin to cook.
Once smooth from the double boiler, transfer it into a large mixing bowl that is room temperature.
Whip on high approximately 15 minutes. Until high foam thickness in a meringue form.
Slowly add in one cube of room temperature butter waiting until it is completely mixed, the meringue will flatten and turn into more a cream base now. Keep adding in the butter until it’s all incorporated.
Whip on high for 2-3 minutes to fluffen up the icing. While mixing add in vanilla, you may also add in other desired flavoring. I recommend staying away from oil-based flavors. You can even add in a little bit of melted chocolate if you want a richer flavor.
If while mixing in the butter your icing went a tad soupy, set in the fridge for 5-15 minutes and whip again. All is not lost.
Mix all the ingredients, even you can add melted chocolate for more amazing flavor, whip it and use this sugar-free buttercream icing for your deserts.