Sponge Cake with Mixed Berries

For Sponge Cake Roll:

Separate two of the eggs, placing the yolks in one large bowl and the whites in another bowl. Add the two remaining eggs and the additional yolk to the bowl with the yolks. Cover both bowls with plastic wrap and allow the eggs to come to room temperature (about 30 minutes).

Position oven rack in the center of the oven. Preheat oven to 400º F. Line a rimmed 12 by 17-inch jelly roll pan with baking parchment paper. Coat the parchment paper with nonstick baking spray and dust with flour, tapping out any excess flour.

In a small bowl, whisk flour and cornstarch together. Set aside.

Add 1/2 cup sugar to the bowl with the egg yolks; using an electric mixer set on high speed, beat until mixture is pale yellow, thick and fluffy (about 5 minutes). Beat in the vanilla extract. Sift half of the flour mixture over the egg mixture and fold gently using a rubber spatula, until flour is just blended. Sift the remaining flour into the batter and gently fold in.

In a separate bowl, beat the egg whites until foamy. Add the cream of tartar and beat until soft peaks form. Slowly add the remaining tablespoon of sugar and beat until peaks are stiff. Gently fold 1/3 of the whites into the batter to lighten it, add remaining whites.

Pour the batter onto the prepared pan, spreading the batter evenly with an offset spatula. Bake for 8 to 10 minutes, or until lightly golden brown. When done the center of the cake should be springy to the touch.

Dust a clean kitchen towel with confectioners' sugar. Invert cake onto the kitchen towel. Gently remove the parchment paper. Starting at the short end, roll cake in towel. Cool completely on a wire rack, seam side down.

For Mixed Berries:

Rinse the blackberries and pat dry. Rinse and hull the strawberries and slice in half. Place half of the fruit in a food processor with 2 tablespoons of superfine sugar. Pulse until fruit is just crushed. Place the crushed fruit in a bowl and combine remaining fruit. Cover and chill for up to 2 hours.

To Assemble: