In a medium saucepan, melt unsalted butter over medium heat.
Add flour and sea salt, stirring continuously until a smooth paste forms.
Incorporate dark brown sugar, ensuring a thorough blend.
Cook over medium heat, stirring constantly, until the mixture bubbles and takes on a rich, caramelized hue, approximately 5 minutes.
Remove the saucepan from the heat.
Gradually stir in boiling water, then return the saucepan to heat.
Bring the mixture to a boil while stirring constantly until it becomes smooth and thickened.
Add pure maple syrup and vanilla extract, stirring until well combined.
Serve the sauce while still warm.
