In a heavy-bottomed 8-quart pot (or larger), gently simmer the whey over low heat until it has reduced by approximately half. This may take 1-3 hours. Stir occasionally, and skim off any foam that rises to the surface. Skimming the foam will remove impurities and improve the flavor and texture of your caramel.

Once the whey is reduced, add the sugar and stir until dissolved. Return to a low simmer.

Continue to simmer, stirring occasionally. The mixture will begin to darken and thicken as it caramelizes. Pay close attention to the color change.

Continue simmering until the caramel reaches a thick, slow-dripping consistency, or when a small amount dropped onto a cold plate forms a soft, sticky ball.

Remove the pot from the heat and immediately stir in the cold butter. The cold butter will stop the cooking process and create a smooth, rich texture.

Allow the caramel to cool slightly, then stir in the sea salt and vanilla extract.

Pour the caramel into your prepared storage containers.

Caution: Caramel is extremely hot. Use care when handling. Refrigerate for a longer shelf life.