Place a silicone trivet or folded towel on the counter.
In a large heavy pot, combine sugar, water, and lemon juice. Make sure all of the sugar is moistened with the water.
Clip thermometer to the pot and heat mixture over high to medium-high heat.
Bring mixture to rolling boil - do not stir
Allow the mixture to boil until thermometer reaches 330°
Be careful not to stir. If it starts to color unevenly you can swirl the pot a little to even it out.
Once the mixture reaches 330° remove from heat and place on trivet or folded towel. Be very careful as this mixture is extremely hot.
Pour cream into the pot. It will bubble vigorously. This is why you need to use a large pot.
When it stops bubbling, wish in vanilla and salt.
Return to medium heat and continue to whisk until mixture comes together and is a caramel color.
Remove from heat and pour into a heatproof bowl and cool completely.
Store in an airtight container for up to one month