Pour the whey into a heavy-bottomed pot. Turn the heat to medium and let the whey reduce to about half. This could take an hour or so. The higher the heat, the more you need to keep an eye on it so it doesn't burn.
8 cups whey
Once the whey is reduced, pour the sugar into the pot with the whey. Stir and cook until the sugar is dissolved.
2 cups cane sugar
Once the sugar is dissolved, let the mixture cook until it starts to caramelize. You should look for a syrup-thick texture and an amber color. This could take another hour. Stir and keep an eye on it.
Once the texture is to your liking, add the butter. Stir until melted.
1/2 cups 1 stick butter
Remove from heat and add int he vanilla extract and salt.
1 tbsp vanilla extract, 1 generous pinch of salt
Use right away or store in an airtight container. You can refrigerate, but you may need to let it come to room temperature before pouring.