I made this with Belle when I visited her last weekend. It was much appreciated by her other guests despite the unintentional addition of an extra ingredient. I'm sure that it would be even better if you don't accidentally pour a dollop of boiling water into the batter.
2 tbsp unsalted butter , softened
1/2 cup plus 2 tbsp sugar
1/8 teaspoon salt
3 large egg yolks
3 tbsp flour
2 - 3 tsp lemon zest
1/4 cup lemon juice
1 cup whole milk
4 large egg whites , at room temperature
Preheat oven to 325 degrees. Grease your pan. (We used a 9 inch square pan, but you could use other shapes or custard cups.) Lay a folded dish cloth in the bottom of a roasting pan and put the greased pan inside. Boil a large kettle of water.
2. Mash 2 tablespoons butter together with sugar and salt until the mixture is crumbly. Beat in the yolks, then the flour, mixing until smooth. Slowly beat in lemon zest and juice, then stir in milk. Beat egg whites to stiff, moist peaks. Gently whisk whites into batter just until no large lumps remain.
3. Immediately ladle (do not pour! I don't know why, but this is important!) the batter into the pan. Set the roasting pan on oven rack. Pour enough boiling water into the roasting pan to come halfway up the sides of the pan with the batter. Do not pour any water into the batter or that side of the cake will be decidedly more pudding-y than the other side. Bake until the cake center is set: 25-30 minutes. Remove the roasting pan from the oven and let the cake stand in the water bath for 10 minutes. Serve and devour.