In a medium saucepan over medium-low heat, melt butter completely.
4 tablespoons unsalted butter
Add brown sugar and stir just a little. Then, let sit until partially melted, about 3 minutes. Keep the stirring to a minimum to avoid crystal formation.
3/4 cup light brown sugar
Add heavy whipping cream and whiskey and stir once to completely incorporate, then bring to a boil until the mixture reaches 225 F on a candy thermometer (about 5 minutes).
3/4 cup heavy whipping cream, 1/4 cup whiskey
Remove from heat and stir in vanilla extract and salt.
1 teaspoon vanilla extract, pinch salt
Cool, store in an airtight container, and refrigerate for up to three weeks or freeze for up to three months.
Notes
You can use dark or light brown sugar. I use light brown sugar; dark brown sugar will yield a sauce with a slightly more molasses flavor and a darker color.I always use unsalted butter and add salt to taste, but if you have salted butter in the house, you can use that and leave off the extra salt.You'll need a candy thermometer. Getting the temperature just right is important to ensure that your sauce has the correct consistency and doesn't burn.