In a heavy saucepan, melt together the butter and brown sugar, stirring constantly to prevent sugar from burning on the bottom of the pan.
Once the butter and sugar have melted, whisk in the heavy cream until smooth.
Cook the sauce over medium heat for 2 to 3 minutes, just until it begins simmering. Continue to stir constantly.
Remove from heat and carefully stir in the vanilla extract (mixture will bubble up) and a pinch of salt, if desired (mixture will bubble up for a moment and then subside).
Cool to room temperature, toffee sauce will thicken as it cools.