Prepare the pan and oven

• Line a 28 x 28 cm cake pan with baking paper.
• Preheat the oven to 170 °C.

Make the yolk batter

• Whisk the milk and vegetable oil together until emulsified.
• Sift in the cake flour and whisk until smooth.
• Add the egg yolks and whisk to a glossy, lump-free batter.

Whip the meringue

• Beat the egg whites with the lemon juice until foamy.
• Add the granulated sugar gradually and whip to soft-medium peaks.

Fold the batters together

• Fold a third of the meringue into the yolk batter to loosen it.
• Fold in the remaining meringue gently with a whisk or a spatula until just combined.

Bake the cake

• Pour the batter into the prepared pan and smooth the surface.
• Tap the pan a few times on the counter to release air bubbles.
• Bake until golden and springy to the touch.

Cool and use

• Let the cake cool slightly in the pan, then transfer to a rack.
• Use as a soft cake base or roll it up for a Swiss roll.