Preheat oven to 325°F (160°C). Line your cake pan with parchment and wrap the outside in foil.
Prepare a water bath by placing a baking dish filled with hot water on the lower rack.
Separate eggs. In one bowl, whisk yolks with Greek yogurt, sugar, flour, cornstarch, milk, vanilla, and lemon juice until smooth.
In another bowl, whip egg whites to stiff peaks using a mixer.
Fold ⅓ of the whites into the yolk batter to loosen, then gently fold in the remaining whites until just combined.
Pour batter into the cake pan and gently tap to release air bubbles.
Place pan in the water bath and bake at 325°F for 25 minutes. Lower heat to 300°F and bake another 30–35 minutes.
Turn off oven, crack the door, and let it rest for 15–20 minutes.
Remove, cool fully, then chill for at least 2 hours.
Dust with powdered sugar and serve with berries, if desired.