In a small saucepan, melt your butter over medium heat, whisk in the cream of tartar and continue whisking as this mixture begins to boil.
Keep whisking as bubbles form, it will take about 2-3min for this mixture to turn a deep golden brown colour. As soon as you see it turning brown, remove it from the heat and continue stirring. You want the butter browned but not burnt.
Whisk your heavy cream and sweeteners into the browned butter.
Turn your heat to low and place the saucepan back over the burner. Continue to whisk until this mixture starts to boil, whisk a few more times, then remove it from the heat.
Mix in your vanilla extract, molasses and xanthan gum and whisk until smooth.
Place this mixture back on the low heat for 1 more minute, whisking often, then shut off the burner and remove your pot from the heat.
Use a spatula to scrape the caramel sauce into a measuring cup or small bowl and place in the fridge to cool.