Preheat oven to 190°C/170°C fan-forced. Grease base and side of a 30cm x 19cm 1.5L-capacity baking dish.
Place butter, sugar, milk, zest, flour and egg yolks in a large bowl. Whisk until well combined and smooth.
In a stand mixer fitted with the whisk attachment, whisk egg whites until soft peaks form. Fold one-third of the egg whites into the lemon mixture until very well combined, then gently fold in the remainder until just combined. Pour into the prepared dish. Sprinkle with extra ¼ cup (55g) sugar. Combine lemon juice and boiling water in a jug and slowly pour over the back of a large spoon onto the batter. Bake for 35 minutes, until just set and golden.
Dust with icing sugar and serve with whipped cream and extra lemon zest.
