In a saucepan on medium-high heat, add butter, sugar, heavy cream, espresso powder, vanilla, and salt. Bring to a boil. Then, reduce the heat, and simmer for 1-2 minutes.
3/4 cup unsalted butter, 1 1/2 cups packed brown sugar, 2 teaspoons vanilla extract, 1 1/4 cups heavy whipping cream, 2 teaspoons instant espresso powder, 1/2 teaspoon sea salt