Gather and measure prune juice, unsalted butter, sea salt, and vanilla extract before beginning.

Pour 1 1/2 cups prune juice into a small saucepan and heat over medium-high until it reaches a gentle boil, about 2 to 3 minutes.

Reduce heat to medium and simmer the prune juice, stirring occasionally, for 5 to 6 minutes until it thickens and reduces slightly.

Add 1/2 cup unsalted butter, cut into small pieces, to the simmering prune juice.

Whisk the butter into the prune juice continuously until fully melted and the mixture is glossy and smooth.

Stir in 1/4 teaspoon sea salt and 1/2 teaspoon vanilla extract until evenly combined.

Remove the saucepan from heat and let the spread cool for 2 to 3 minutes.

Transfer the warm spread into sterilized glass jars and let cool completely before sealing and storing.