Preheat oven to 350°.
Lightly grease 6 ramekins.
Bring water to a simmer on the stove, enough to halfway-fill the roasting pan later. A tea kettle works great for this.
Add the butter and 3/4 cup of the sugar to a large bowl. Mix until creamy.
Add egg yolks one at a time, beating after each addition.
Mix in the lemon zest.
Add the flour and salt and mix until blended.
Add the milk and lemon juice and mix until well combined. Set aside.
Get a fresh bowl that is squeaky clean and free of grease. Add the egg whites and beat with a hand mixer until whites reach the soft-peak stage.
Add the remaining 2 tablespoons of sugar and beat until stiff peaks form.
Gently fold about 1/4 of the beaten egg whites into the lemon/flour mixture. Carefully fold in the remaining egg whites in two more additions, until whites are completely mixed in. The mixture may be a little runny.
Divide the batter equally among the ramekins. You may have batter left over.
Set the ramekins into a roasting pan. Gently pour enough hot water into the roasting pan to come halfway up the sides of the ramekins.
Bake on the center oven rack until cakes are deep golden but with a still slightly soft center, 25-30 minutes.
Remove from oven and cool on a rack for 10 minutes before transferring to the fridge. Let chill uncovered for four hours. Dust with powdered sugar (optional) before serving. Store covered in the refrigerator.