Add the sliced butter to a 2 to 3-quart saucepan over medium-high heat. Allow the butter to melt completely.
Whisk in the dark brown sugar, dark Karo syrup, vanilla, ground cinnamon, ground nutmeg, and kosher salt. Whisk constantly until the brown sugar mixture reaches a low boil and continue whisking for about 4-5 minutes. Remove from the heat.
Immediately whisk in the heavy cream. (The brown sugar sauce will be a bit thin but will thicken as it cools) Allow the brown sugar sauce to cool completely.
Notes
If you prefer, you can remove the ground cinnamon and nutmeg from the recipe. The sauce will still be delicious without these optional ingredients.