Preheat your oven to 350°F (175°C). Butter a 2-quart baking dish or individual ramekins.
Cream the butter and sugar together in a bowl until light and fluffy.
Add the egg yolks one at a time, beating well after each addition.
Mix the lemon zest and juice into the butter mixture.
Sift the flour, baking powder, and salt together. Gradually add this to the wet mixture, alternating with the milk. Start and end with the dry ingredients. Mix gently until smooth.
Beat the egg whites in a clean bowl until stiff peaks form.
Fold one-third of the egg whites into the batter to lighten it. Then gently fold in the remaining egg whites, being careful not to deflate the mixture.
Pour the batter into the prepared baking dish or divide among ramekins.
Bake for 35–40 minutes (20–25 minutes for individual ramekins) until the pudding has risen and is golden on top.
Serve hot, straight from the oven. It’s delicious on its own or with a dollop of cream or custard.