In a medium saucepan over medium heat, add the unsalted butter. Allow it to melt completely, swirling the pan occasionally to ensure it doesn’t brown.
Once the butter is melted, add the dark brown sugar to the saucepan. Stir continuously until the sugar is combined and the mixture is smooth.
Slowly pour in the heavy cream, whisking constantly. The mixture will bubble up, so be careful! Continue stirring until everything is well combined.
Remove the saucepan from heat and stir in the fleur de sel and vanilla extract. Whisk until completely incorporated.
Let the sauce cool slightly before transferring it to your jar or container. Allow it to cool to room temperature before sealing.
