1 tablespoon unsalted butter, softened
2 tablespoons all-purpose flour
8 eggs, separated
1½ cups granulated sugar plus ½ cup for sprinkling
1¼ cups potato flour or potato starch
1 (12-ounce) jar strawberry jelly or jam
1 pint (2 cups) heavy cream, whipped to soft peaks
1. Preheat the oven to 350 degrees. Line a 13 × 9-inch baking sheet with parchment paper. Lightly grease the paper with the butter, and then dust with the flour. Whisk the egg yolks and 1½ cups sugar with an electric mixer or beater on high until the mixture turns pale in color, about 3 minutes. Whisk the egg whites with an electric mixer or beater on high until soft peaks form.
2. Fold half the egg whites into the yolk and sugar mixture, followed by the potato flour, and then the remaining half of the egg whites. Gently pour the mixture into the prepared pan and smooth out to the corners with an offset spatula. Bake for 10 to 15 minutes in the center of the oven. Check to see if the sponge cake is cooked by gently pressing the top; it will spring back when done. Have ready a sheet of parchment paper (at least 13 × 9-inches) sprinkled with the remaining ½ cup granulated sugar. Turn the sponge cake out onto the sugared parchment paper and peel off the baking parchment. Allow the cake to cool. Spread the cake lightly with the jam and whipped cream. Roll the cake into a tight spiral, using the parchment paper so not to touch the cake. Slice and serve.
MAKES 8 SERVINGS