To make the matcha sponge, separate the eggs, putting the whites and yolks into separate large bowls. Whisk the yolks with half of the sugar until thickened and pale. Add in the flour, cornflour, matcha and milk, and mix until smooth and fully combined. Whisk the egg whites, sprinkling in the remaining sugar as you whisk until the whites are glossy and just holding soft peaks. Add about 1/3 of the whites to the yolk mixture, folding to combine. Repeat with the remaining whites in two more additions, folding until just evenly combined and no streaks remain.