First, zest and juice your lemon. Set aside half of both – you’ll use half for the sauce, and half for the cake.
Set a medium sized saucepan on the stove over a medium high setting. You want it to be warm before you add the sugar.
When it’s warm, add the sugar, and stir it around the pot with a wooden spoon for 1 or 2 minutes to heat it all the way through.
Pour in the coconut milk, and add the sea salt, stirring to blend together thoroughly.
Add half the lemon juice and zest, and continue to stir. (You'll use the rest of the zest in the sauce).
Heat until the mixture comes to a boil, and then reduce to a medium low setting so that it remains at a simmer. Allow the sauce to simmer for about 15 minutes, until it has thickened. Stir occasionally.
When it’s done, it will stick to the back of your spoon.
Remove from the heat, and transfer to a covered container, preferably with a lid you can seal.
Refrigerate for at least two hours. It will continue to thicken as it cools.