01 -
Preheat the oven to 230°F (110°C). Line the bottom and sides of a 6-inch (15 cm) round cake pan with parchment, letting the side parchment rise above the rim. Place the pan in a larger roasting pan.
02 -
Whisk the egg yolks with 25 g of the sugar and the lemon zest until smooth. Add the Greek yogurt and vanilla if using, then whisk in the flour and cornstarch until no dry streaks remain.
03 -
In a clean bowl, beat the egg whites with the lemon juice until foamy. Add the remaining 40 g sugar a little at a time and beat to glossy medium peaks.
04 -
Fold one-third of the whipped whites into the yogurt batter to loosen it. Fold in the remaining whites in two additions, using gentle strokes from the bottom of the bowl.
05 -
Pour the batter into the prepared pan. Add hot water to the roasting pan until it reaches about 1 inch up the side of the cake pan. Bake for 50 minutes at 230°F (110°C), then raise the oven to 340°F (170°C) and bake 8-10 minutes more until the top is set.
06 -
Turn off the oven and crack the door for 10 minutes with the cake inside. Remove the pan, cool to room temperature, then chill for at least 2 hours before unmolding and slicing.
07 -
Dust with powdered sugar and serve with fresh berries if desired. Slice with a warm, clean knife for neat pieces.