Preheat oven to 350 degrees F. and butter four 6-ounce ramekins and place in a baking pan with a 2-inch lip.
With electric beaters, beat butter and sugar until pale and fluffy. Add the lemon zest and juice and then add the egg yolks, one at a time (don’t worry if the mixture curdles – it’ll work out fine). On low speed, add in flour and baking powder, then milk a little at a time, until mixture is evenly blended.
Whip egg whites until they hold a stiff peak and gently fold a third of the whites into the lemon mixture. Fold in the remaining two thirds of whites in and spoon batter into prepared ramekins. Pour boiling water around the ramekins to come halfway up and bake for 35 minutes, until top of puddings are risen and a rich golden brown. Serve warm.