Add ice and cold water to a large bowl, preferably a metal one, to create an ice water bath.
Cut the lemon into rounds as thinly as you can. Remove any seeds.
Add water to a saucepan and bring to a full boil.
Remove from the heat and add the lemon rounds to the hot water. Let them soften for about 2 minutes, stirring gently once or twice.
Drain the saucepan and then dip the lemon rounds into the ice water bath for a minute. Drain, and place the lemon rounds on a plate.
Add the sugar and 2 cups water to a heavy skillet. Bring to a boil and stir until the sugar is dissolved.
When the mixture is clear, reduce the heat to a low setting and add the lemon rounds and vanilla beans.
Heat without boiling for about 1 hour. The lemon rinds should be translucent.
Line a baking sheet with parchment paper, and, using tongs, transfer the lemon rounds in a single layer. Allow them to cool to room temperature.
Once the lemon rounds are cool, create your own double boiler by filling a medium sized saucepan about half full with water. Bring to a boil. Now place the chocolate into a heat-proof bowl and place it over the saucepan with water in it. Stir occasionally as the chocolate melts.
Remove the bowl from the hot saucepan, and dip the lemon rounds half into the melted chocolate.
Allow to cool on the parchment paper.
Sprinkle with sea salt as the chocolate cools.
Refrigerate for at least 1 hour before serving.