Step 1

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

Step 2

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.

Step 3

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.)

Step 4

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.

Step 5

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.

Step 6

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.

Step 7

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.

Step 8

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).

Step 9

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