Step 1

Preheat the oven to 325°F. Line an 8x8-inch baking dish with parchment paper, leaving some parchment paper overhang for lifting the cake out of the pan. Set it aside.

Step 2

Using either a stand mixer or a medium-sized mixing bowl and a handheld mixer on high, whip together the egg whites and the white vinegar to stiff peaks.

Step 3

Using a large mixing bowl and a handheld mixer on medium-high, beat together the egg yolks and granulated sugar for 2 to 3 minutes until pale yellow.

Step 4

Lower the mixer speed to low. Drizzle in the melted and cooled butter and vanilla.

Step 5

Keeping the mixer speed on low, beat in the flour. Mix just until well combined.

Step 6

Continue mixing on low, and add in the lemon juice and zest.

Step 7

Keep the mixer speed on low and drizzle in the milk.

Step 8

Using a whisk, incorporate the egg whites into the batter. Leave small egg white bumps.

Step 9

Pour the batter into the prepared baking dish; it will be thin.

Step 10

Bake for 45 to 50 minutes, just until the mixture has no wobble.

Step 11

Allow the cake to cool completely. Dust with powdered sugar and slice.