Pour the cranberry juice into a heatproof glass or bowl. Warm to touch in the microwave.

In a pot, add the sugar and a squeeze of lemon. Stir to mix in the lemon. Turn the heat on and begin cooking the sugar. It will melt and turn brown as it does, stir with a wooden spoon to keep it caramelizing evenly and press out any sugar lumps. Most of the lumps will dissolve on their own as the sugar melts and caramelizes. Once it’s all melted let it fully caramelize for a minute. Slowly pour the cranberry juice in, stirring as you do. Go slowly or the caramel will seize. Once it’s all in add the butter and stir until the butter is melted and the sauce is smooth. Stir in the vanilla and heavy cream if using. Let cool. Store in an airtight container at room temperature.