Combine the sugar, water, and lemon juice in a heavy saucepan. Bring to a boil and gently stir to dissolve the sugar. Using a candy thermometer attached to the side of the pan, bring the mixture to the caramel stage: 320°F. * Remove from the heat.
Bring the heavy cream to a boil and add a few tablespoons at time into the caramel mixture. Be careful because it will bubble up and spill out if you add it too quickly.
Stir in the salt** then swirl in the tablespoon of butter slowly. Let the sauce cool completely before transferring to an air tight container. Refrigerate until ready to use. Let it sit at room temperature for 30 minutes before using.
*As the temperature nears the end watch that the color doesn't get too dark. It goes quickly from light brown to burnt.
**The sauce thickens significantly as it cools. Don't add extra cream or transfer it to the heat to thicken it until the sauce is room temperature and can be gauged accordingly