Preheat your oven to 350°F.
Line a square baking tin.
Mix the milk with the lemon juice and set aside to thicken.
Rub the lemon zest into the sugar to enhance the flavor.
Combine the sugar mixture with the rest of the wet ingredients in the milk mixture. Sift in the dry ingredients.
Gently mix until just combined.
Pour the batter into the prepared tin.
In a heatproof bowl, mix the lemon juice with the cornstarch to form a smooth paste. In a separate jug, combine the caster sugar with the boiling water, then pour it over the cornstarch mixture and whisk until smooth.
Pour this lemon sauce over the sponge batter.
Bake for 45-50 minutes, until golden and set, and the sponge springs back when touched.
Serve with a dusting of powdered sugar and a side of custard.