Combine the butter, brown sugar, heavy cream, and salt in a medium, non-stick saucepan over medium heat.
1/2 cup butter, 1 cup dark brown sugar, 3/4 cup heavy cream, 1/2 teaspoon salt
Without stirring, allow the mixture to heat up and the butter to melt completely. Let it come to a simmer.
Once simmering, let the mixture cook at a steady boil for 4 to 5 minutes, stirring occasionally with a silicone spatula to prevent sticking.
Remove the saucepan from the heat. Stir in the vanilla extract.
1 teaspoon vanilla extract
Let the sauce cool in the pan for 10-15 minutes before transferring it to a jar for storage. It will thicken as it cools.
Notes
Keep the heat steady, using medium heat. If the heat’s too high, the sugar may burn. However, if it's too low, it may not thicken enough.
Don't stir too soon. Let the butter and sugar melt together undisturbed at first. Stirring too early can cause sugar crystals to form and make the sauce grainy.
Add the vanilla after removing the saucepan from the heat. It'll keep the flavor strong and keep it from cooking off.
Use a non-stick pot and silicon spatula for easy cleanup. It also helps prevent the sugar from scorching on the bottom of the pan.
Use a saucepan that's a little larger than you think you'll need. The mixture will foam up considerably as it boils and a larger pot will help keep it from spilling over.
This recipe makes about 2 cups of butterscotch sauce