For the caramel - combine 1 ¼ cups of sugar and the juice of ½ lemon in a small, heavy-bottomed pot.
Add just enough water to moisten to a sandy texture. Brush any residual sugar off the sides of the pot with a pastry brush dipped in water.
Cook over medium-high heat until the sugar caramelizes. Remove from heat and, standing at arm’s length from the pot, add the ¼ cup of water. Be careful: The mixture will bubble and sputter. When the sputtering stops, stir and pour the caramel into a glass mold. Carefully tilt the mold to coat with caramel.
For the custard - soak the gelatin granules in 4 tbl. cold water for 15 minutes, until softened.
Pour the whipping cream into a saucepan and whisk in the sugar.
Heat to just below a boil.
Add the softened gelatin and stir until it dissolves completely.
Stir in the buttermilk and lemon juice.
Remove from heat and let cool.
After cooling, pour the custard into the caramelized mold. Refrigerate overnight, until the gelatin sets and the custard is stiff enough to unmold.
To unmold, slip a sharp knife around the inside of the mold to loosen the custard. Invert the mold over a serving plate and gently slide the panna cotta out of the mold.