1 teaspoon vanilla extract or vanilla bean paste, divided
6 tablespoons unsalted butter softened
1/2 lemon finely grated zest
3/4 cups milk whole or lowfat, or buttermilk, well-shaken
1 1/2 cups all-purpose flour
2 teaspoons baking powder
1 pound fresh strawberries halved or sliced
1/2 lemon juice
2 tablespoons granulated sugar divided
1 cup heavy cream
2 tablespoons sour cream or creme fraiche
1 teaspoon vanilla extract or vanilla bean paste
Confectioners’ sugar for dusting (optional)
mint leaves for garnish (optional)
Equipment
Stand mixer with whisk attachment or hand mixer
9×13-inch cake pan
Toothpick
Parchment paper
Cooling rack
Offset spatula
Directions
Heat oven to 350°F (175°C). Line the bottom and sides of a 9×13-inch cake pan with parchment, and coat it lightly with nonstick cooking spray.
In the large bowl of a stand mixer fitted with the whisk attachment, beat the egg whites with 1/4 teaspoon salt on medium/low speed until they begin to thicken. Increase the speed to medium, and add half the sugar — 3/4 cup — a little at a time, letting each sprinkle disappear before adding more. Add 1/2 teaspoon vanilla extract or paste and continue to whip the mixture until the egg whites are glossy, and stiff peaks form when the whisk is lifted. Set this bowl aside.
In a second bowl, beat the butter with the remaining 3/4 cup sugar, remaining 1/4 teaspoon salt, remaining 1/2 teaspoon vanilla, and lemon zest until light and fluffy. Add egg yolks and beat to combine. Sprinkle batter with baking powder and beat thoroughly. Add the milk and beat to combine. Add the flour and beat until smooth.
Spoon the cake batter into the prepared cake pan and spread it into a thin, even layer. Dollop the egg white mixture all over the batter and gently spread this across the cake. Bake the cake for 20 minutes. Check for doneness— a toothpick inserted should come out batter-free. The cake might need 5 to 10 minutes longer to set.
Once baked, cool the cake in the pan for 10 minutes, then use the parchment to carefully transfer the cake onto your cooling rack to cool completely.
In a medium bowl, combine strawberries, 1 tablespoon of sugar, and lemon juice and set aside for 15 minutes until juicy. In a large bowl, combine heavy cream, crème fraîche, remaining 1 tablespoon sugar, and vanilla extract or paste, and beat until soft peaks form.
Carefully cut the cooled cake in half the long way, creating two long rectangles. Transfer the first half onto a plate. Swirl the top with half the whipped cream, then scatter with half the berries. Place the second half of the cake on top and finish with remaining cream and berries. Drizzle any extra juices over the cake. Garnish with mint leaves and/or powdered sugar if desired.
Store the cake in the fridge until needed. Cut the cake into slices to serve. Leftovers keep in the fridge for up to 4 days.