BERRY CHANTILLY SHEET CAKE

Steps:

• For the cake: Place a rack in the center of the oven and preheat to 350 degrees F. Grease the bottom only of a 9-by-13-by-2-inch pan with butter or shortening. Line the bottom only with a piece of parchment.
• Place the egg whites in the bowl of an electric mixer fitted with the whisk attachment and whip on medium-high speed until frothy, about 1 minute. Slowly stream in 3/4 cup of the granulated sugar and continue to whip until medium-stiff peaks form, about 3 minutes. The meringue will hold its shape and have the consistency of shaving cream.
• Transfer the meringue to a large clean bowl and set aside. Wipe out the mixer bowl with a paper towel. Sift the cake flour, remaining 3/4 cup white sugar, baking powder and salt into the same mixer bowl. Add the lemon zest and whisk to combine. Add the vegetable oil, buttermilk,
• vanilla and egg yolks and blend on low until combined. Scrape the bowl and then mix on
• medium speed for 1 minute.
• Remove the bowl from the mixer. Use a large whisk or spatula to fold one-third of the meringue into the flour mixture until lightened and a few streaks of egg white remain. Repeat with half the remaining meringue, folding into the batter until nearly combined. Fold the last third of the meringue into the batter, folding just until the meringue is completely incorporated.
• Pour the batter into the prepared pan. Tap the pan gently on the counter 3 to 4 times to release any large air bubbles. Bake until a toothpick inserted in the center of the cake comes out with just a few moist crumbs clinging to it and the cake is golden brown, 33 to 36 minutes.
• Place the pan on a cooling rack and cool 20 minutes. While still warm, cover the pan with plastic and allow to cool completely, about 4 hours, before frosting.
• Remove the plastic wrap and turn the cake out onto a large serving platter or sheet pan. Peel off the parchment if it is still stuck to the bottom of the cake.
• For the whipped cream: Combine the mascarpone, confectioners' sugar, vanilla and salt in the bowl of an electric mixer fitted with the whisk attachment. Mix on low until combined, then raise to the speed to medium and beat for 30 seconds or until smooth. Scrape the bowl and beat for another 30 seconds.
• Lower the mixer to low speed and slowly pour in the cream. Once the cream is combined, stop and scrape the bowl and then raise the speed to medium-high and beat until medium-stiff peaks form, 2 to 3 minutes. The cream will hold its shape but will still be spreadable.
• Spread the jam evenly over the top of the cake with an offset spatula, stopping just short of the edge, creating a thin, see-through coating. Spoon the whipped cream onto the cake and spread it evenly to the edges. Use a large knife to mark 3 rows of 4 slices for a total of 12 slices. Arrange a small handful of mixed berries in the center of each marked piece. Cut and serve, storing any leftovers covered in the refrigerator for up to 2 days.