Begin with a mixture of water, sugar, and lemon juice (if using) in a medium saucepan, placed on low heat. Stir gently until the sugar dissolves.

Once the sugar has dissolved, turn the heat up to medium heat and stop stirring to avoid the unwanted crystallization of the melted sugar. Use a wet pastry brush to brush down any stray sugar crystals stuck to the sides of the pan. 

Let the mixture boil until it begins to change color and caramelize. You can occasionally place a lid on the saucepan, creating condensation that will push down any sugar crystals forming on the side. As the water evaporates, your sugar will take on a caramel hue. You can gently swirl the pan every so often to ensure even cooking. 

Whisk in the butter cubes once the caramel reaches a deep amber or desired color. You don't want it too light, or it won't have flavor, nor too dark, or it'll be bitter.

Next, whisk in the heavy cream. The mixture will bubble quickly and rise, so please take care. Take it off the heat and stir through salt and vanilla.

Leftover caramel can be stored in the refrigerator in an airtight container for up to a month.