[Read Nancy Harmon Jenkin's article "Bergamot."]
100 gr (¼ lb) unsalted butter, at room temperature
50 gr (½ cup) sugar
pinch of sea salt
4 medium eggs
125 ml (½ cup) freshly squeezed lemon or bergamot juice
grated zest of 2 lemons or bergamots (3 to 4 tsp)
Fill the bottom half of a double boiler about three-quarters full of water — so th…