Preheat oven to 350 degrees. Butter bottom and sides of an 11-by-17-inch rimmed baking sheet. Line pan with parchment paper and butter again. Dust with flour; tap out excess. In a small bowl, sift together flour and cornstarch.

In the bowl of an electric mixer fitted with the whisk attachment, beat egg yolks, 3/4 cup granulated sugar, vanilla, and salt on high speed until thick and pale, about 3 minutes. Transfer egg-yolk mixture to a large bowl. Wash and dry mixer bowl and whisk attachment.

Place remaining egg whites in mixer bowl, and whisk on medium speed until soft peaks form, about 1 1/2 minutes. With mixer running, slowly add remaining 6 tablespoons granulated sugar; continue whisking until stiff and glossy, about 1 minute more.

Fold egg-white mixture into egg-yolk mixture. In three additions, sift reserved flour mixture into egg mixture, folding to combine after each addition. Transfer batter to prepared baking sheet, smoothing top with an offset spatula. Bake until light golden brown and a cake tester inserted in the center comes out clean, about 20 minutes.

Run a knife around the sides of cake. Invert onto a kitchen towel dusted with confectioners' sugar; remove parchment. Lightly dust with confectioners' sugar using a sieve. Using the towel and starting at a short end, roll up cake and towel. Let cool completely, seam side down.

Unroll cake. Whip cream until soft peaks form; mix in jam. Spread on top of cake, leaving a 1/2-inch border. Starting at a short end, roll up cake without towel. Wrap jelly roll cake in towel. Transfer to baking sheet, seam side down to maintain cylindrical shape. Refrigerate at least 30 minutes and up to 3 hours. Dust with confectioners' sugar before slicing and serving.