Bring cream, butter and salt to a boil in a small saucepan. Remove from heat and set aside.
In a medium saucepan, bring sugar, agave and water to a boil, stirring just until the sugar is dissolved. Then continue boiling without stirring until the mixture is a light golden color and a candy thermometer reaches 250 degrees. Don’t worry how long this takes – some recipes say it takes 10 minutes and others say up to 45 minutes. The important thing is that it reaches 250 degrees and is a light golden caramel color.
When this happens, carefully stir in the butter/cream mixture. The caramel will bubble up, so be careful. Stir constantly and continue to cook until the mixture reaches 248 degrees.
Then pour into the prepared baking dish and let cool at least 2 hours (LF note: I probably let it go more like 3 or 4. It's easier to slice that way). Sprinkle the top of the caramels with a generous amount of nice sea salt. When the mixture has set, cut into pieces and wrap in parchment paper. (LF note: try to to handle the caramels to much when cutting a wrapping since the body heat from your hands will soften them).