Instructions

for the strawberry filling

Cook strawberries and sugar in large saucepan over medium heat, mashing occasionally with potato masher, until thick and jamlike, 10 to 15 minutes (mixture should measure 2¼ cups). Meanwhile, combine water, lemon juice, and gelatin in small bowl and let stand until gelatin softens, about 5 minutes. Stir gelatin mixture into hot strawberry mixture until dissolved. Transfer to large bowl and refrigerate until set, at least 4 hours.

 for the cake

Adjust oven rack to lower-middle position and heat oven to 350 degrees. Grease 18 by 13-inch rimmed baking sheet, line with parchment paper, grease parchment, and flour sheet. Whisk flour, baking powder, and salt together in bowl; set aside.
Using stand mixer fitted with whisk attachment, whip eggs at medium-high speed and gradually add granulated sugar and then vanilla, about 1 minute. Continue to whip mixture until very thick and voluminous, 4 to 8 minutes. Sift flour mixture over egg mixture; using rubber spatula, gently fold batter until just incorporated.
Pour batter into prepared sheet with rubber spatula and spread into even layer with offset spatula. Bake until cake is firm and springs back when touched, 12 to 17 minutes, rotating sheet halfway through baking. Lay dish towel on counter and dust with confectioners’ sugar.
Immediately run thin knife around edge of baking sheet to loosen cake, then flip hot cake out onto prepared dish towel. Gently peel off parchment and discard. Starting at short edge, roll cake and towel into log and cool completely, seam side down, about 1 hour.

for the mascarpone mixture

Using handheld mixer fitted with whisk attachment, whip cream, mascarpone, sugar, lemon juice, and vanilla on medium speed until stiff peaks form, about 3 minutes. Gently unroll cake and spread with strawberry mixture, leaving 1-inch border around edges. Top with whipped cream and reroll cake snugly. Wrap in plastic wrap and refrigerate until firm, at least 4 hours. Dust with confectioners’ sugar. Serve.