Citrus Meringue Roulade (Citrus brazo de Mercedes)

Preheat oven to 350°F (180°C). Spray a 13×9-inch rimmed baking sheet with baking spray with flour. Line bottom of pan with parchment paper; lightly spray parchment.

In the bowl of a stand mixer fitted with the whisk attachment, beat egg whites, cream of tartar, and salt at medium-low speed until soft peaks form, 5 to 6 minutes. With mixer on low speed, add granulated sugar in a slow, steady stream, beating until combined (about 3 minutes total). Increase mixer speed to medium, and beat until thick, shiny, and no longer gritty when rubbed between 2 fingers and stiff peaks form, 5 to 10 minutes, stopping to scrape sides of bowl halfway through mixing. Add vanilla, and beat at medium-high speed until combined, about 30 seconds. Spread meringue into prepared pan, smoothing top with a large offset spatula.

Bake until meringue is set and top is light golden brown, 20 to 23 minutes. (Meringue will be very puffy in oven but will deflate while cooling.) Let meringue cool in pan for 5 minutes; dust top with1 tablespoon (7 grams) confectioners’ sugar.

Meanwhile, sift remaining 2 tablespoons (14 grams) confectioners’ sugar into a 13×9-inch rectangle on a clean kitchen towel. Using a small knife, carefully loosen edges of meringue from pan, and invert onto prepared towel; gently peel off parchment, and discard. Using a greased serrated knife, gently trim edges.

Working quickly, dollop and spread Citrus Custard Filling on top of meringue, leaving a ½-inch border around edges. Starting at one short side and using towel to help support, gently roll up meringue, jelly roll style. (It’s OK if there are some small cracks in your meringue.) Place, seam side down, on a serving plate; garnish with confectioners’ sugar and orange zest, if desired. Serve immediately.

In a medium heatproof bowl, place a fine-mesh sieve.

In a small saucepan, whisk together condensed milk, egg yolks, lemon zest, orange zest, and salt.

In a small bowl, whisk together 2 tablespoons (30 grams) water and cornstarch until smooth and well combined; whisk into condensed milk mixture. Cook over medium heat, whisking frequently, until thickened and beginning to boil. Cook, whisking constantly, for 1 minute. Strain through prepared sieve into bowl. Cover with a piece of plastic wrap, pressing wrap directly onto surface of custard to prevent a skin from forming, and let stand at room temperature until ready to use.