Melt the Butter: Brown butter in a medium saucepan over moderate heat.
Add Sweetener: Add the erythritol or allulose and mix well. Whisk until the sugar begins to melt and lightly caramelize ~3–4 minutes.
Pour in the Cream: While whisking add the cream gradually as well. This is normal—to also bubble up.
Simmer and Thicken: Turn the heat to low and simmer the sauce for about 8–10 minutes, stirring occasionally until it has thickened and darkened.
Finish with Vanilla & Salt: Remove from heat. Add vanilla and a bit of salt for balance of flavor. By cooling slightly it will also thicken up a lot more on resting.