Preheat oven to 325F (170C) and place rack in the center of the oven. Butter six - 1 cup (240 ml) ramekins or one 9-inch glass pie dish.
Set aside 2 tablespoons (28 grams) of the sugar to use when whipping the egg whites. Place the remaining sugar in a medium sized bowl. Add the lemon zest to the sugar. With the back of a spoon, or spatula, grind the lemon zest into the sugar, creating a fragrant, slightly yellow tinged sugar.
Working the zest into the sugar will release lots of the essential oils in the zest, creating a super lemony batter.
Then, in the bowl of your electric mixer or with a hand mixer, cream the lemony sugar and butter. Add the three egg yolks, one at a time, and beat until incorporated. Beat in the vanilla extract. Add the flour and salt and beat until combined. With the mixer on low speed, gradually pour in the lemon juice and milk. Set aside while you beat the egg whites.
In a clean bowl of your electric mixer, or with a hand mixer, beat the egg whites until frothy. Add the cream of tartar and continue to beat until soft peaks form. Gradually add the remaining 2 tablespoons (28 grams) of sugar and beat until stiff peaks form. Gently fold the egg whites into the batter, in three additions, mixing only until incorporated.
Carefully ladle the batter into the prepared ramekins. (The batter does not rise much during baking so you can fill the ramekins almost to the rim.) I poured the entire mixture into the 9-inch pie pan. Place the ramekins in a larger baking pan (or any size pan that will fit the ramekins and leave about 1 inch (2.54 cm) around the edges). Or place the 9-inch baking dish in a larger basting pan and set on your counter.
Boil a tea pot of water to create a water bath. (A water bath is used to provide temperature protection for the eggs.) Place the basting pan with the souffle inside into the oven, carefully pulling the oven rack out a bit. Carefully pour in enough hot water so that the water is halfway up the sides of the ramekins or pie dish, and carefully slide the rack back into the oven.
Bake for about 40 - 45 minutes or until the sponge cakes are golden brown and a toothpick inserted into the cake portion comes out clean. Be careful not to insert the toothpick into the lemon sauce at the bottom of the ramekins. Remove the ramekins from the water bath and cool slightly before serving.
This dessert can be served warm or at room temperature. Dust the tops of the puddings with confectioners (powdered or icing) sugar and dress with a dollop of whipped cream and fresh fruit (optional).