In a small saucepan, bring the heavy cream to a light boil.
Reduce heat to low and simmer the cream for about 10-12 minutes, uncovered. The cream will reduce in volume and evaporate and foam can form on top. Whisk every 1-2 minutes to prevent the cream from sticking or burning on the side of the saucepan.
Remove from heat and whisk in butter, powdered erythritol, and vanilla extract
When butter is melted, sprinkle the xanthan gum all over the pan to avoid lumps and whisk vigorously to incorporate.
Return the saucepan to medium heat again, keep whisking, and cooking for 3-5 minutes, small bubbles should form on side of the saucepan until liquid thickens. If not thick enough to your liking, sprinkle an extra 1/4 teaspoon xanthan gum all over the pan and keep cooking/stirring for an extra 2 minutes. Keep in mind that the liquid thickens even more when cooling down, so don't add too much extra gum!
Cool down into a glass mason jar for 30 minutes before closing. When it reaches room temperature it will thicken again and it can be stored in the pantry for 2 days then in fridge for up to 5 days. It will harden in the fridge as the butter solidify the mixture. Read my note below for tips
Freeze in ice cube tray in single-serve portions. Defrost one portion at a time at room temperature into a small bowl.
Use as a Coffee creamerIf you intend to use it in your coffee, as a coffee creamer, spoon the condensed milk into a blender and pour the hot coffee on top. Blend on high speed to combine. You MUST blend coffee and condensed milk or you will end up with two layers, coffee at the bottom and oil on top.Rewarm harden condensed milkThis recipe harden in the fridge and that's normal. Bring the jar of condensed milk, or the amount you want for your recipe into a small bowl and microwave 20-30 seconds or rewarm on the stove in a saucepan to bring back to its liquid state. Give a good stir before serving to blend the flavors and remove any thin layer that could form on top of the jar.