Del Posto's Tiramisu

Preheat oven to 375°. Line a rimmed baking sheet with parchment paper.

Using an electric mixer on low speed, beat egg whites in a large bowl until just combined. Slowly add ½ cup sugar, gradually increasing the speed to medium, and beat until a stiff meringue forms. Transfer to a medium bowl; chill until ready to use.

Whip egg yolks, 2 Tbsp. water, and remaining ½ cup sugar in the still-messy bowl you used for the meringue on high speed until a fluffy, ribbony mixture forms. Using a rubber spatula, gently add reserved meringue and fold until just combined. (Be nice. Streaks are fine. The less you manipulate this, the fluffier it will be later.)

Sift flour, cornstarch, and salt into a medium bowl. Gently fold into egg mixture until no dry spots remain. Carefully transfer batter to prepared pan with spatula.

Bake cake until top is golden brown and still soft to the touch, 7–10 minutes. Let cake cool completely in pan. Run a knife around sides of pan to release cake. Place a cutting board on top of pan. Invert cake onto board; remove pan and carefully peel away parchment. Using a serrated knife, cut cake in half crosswise.

Using an electric mixer on low speed, beat mascarpone, sugar, egg yolks, rum, heavy cream, and salt in a large bowl. Gradually increase the speed to medium as mixture becomes stable and beat until soft peaks form, but don’t over-whip.

Spread a thin layer of mascarpone cream in a shallow serving dish. Place one half of sponge cake on top. Using a squirt bottle or spoon, soak cake with ¼ cup espresso. Spread with half of remaining mascarpone cream, place second cake half on top, and soak again with remaining ¼ cup espresso. Don’t be stingy with the espresso, okay? Top with remaining mascarpone cream.

Spoon cocoa powder into a double-layer of cheesecloth, pull together ends tightly to form a ball, and gently shake cocoa onto tiramisu until fully blanketed (alternately, place cocoa powder in a fine-mesh sieve and dust over tiramisu).

Do Ahead: Cover tiramisu and store in refrigerator for up to 8 hours.