1-Preheat the oven to 325°F: Preheat the oven to 325°F. Line 8×8-inch baking dish with parchment paper leaving overhang the sides, lightly grease with cooking spray, then set aside.
2-Whip the egg whites until STIFF peaks: Whip the egg whites until STIFF peaks form, set aside.
3-Beat the egg yolks and sugar until: Beat the egg yolks and sugar until pale yellow.
4-Add melted butter and vanilla and mix: Add melted butter and vanilla and mix until evenly combined.
5-Mix in flour until evenly incorporated: Mix in flour until evenly incorporated.
6-Add lemon juice and lemon zest and: Add lemon juice and lemon zest and mix until combined.
7-Slowly beat in the milk until well: Slowly beat in the milk until well combined.
8-Add egg whites and whisk by hand: Add egg whites and whisk by hand. You don’t need to fold it in completely. Leave small egg whites lumps. When you pour it into the pan, lumps will float on the surface and create the top layer during the baking. NOTES: The batter is very, very thin!!!
9-Pour the batter into the pan and: Pour the batter into the pan and bake for 40-60 minutes (until the cake is barely jiggly in the center, but the top is firm to touch). Baking time might vary depending on your oven or pan you use but start checking after 35 minutes. If the top browns quickly, you can cover the cake with aluminum foil. Do not overbake it, or the cake will turn out rubbery not custardy in the center.
10-Cool the cake completely before dusting it: Cool the cake completely before dusting it with powdered sugar.
11-Store leftovers in the fridge: Store leftovers in the fridge