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1
Whisk together yolks, zest, lemon juice, sugar, and cream of tartar in a 1-quart heavy saucepan.
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2
Add butter and cook over moderately low heat, whisking constantly, until curd is thick enough to hold marks of whisk and first bubbles appear on surface, about10 minutes.
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3
Immediately pour into a bowl, then chill, covered, until cold, about 5 hours.
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4
Preheat oven to 400F.
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5
Butter a 15- by 10- by 1-inch shallow baking pan (jelly-roll pan), then line bottom lengthwise with a large sheet of wax paper, letting paper hang over ends by 2 inches, and butter paper.
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6
Beat egg whites in a large bowl with an electric mixer until frothy.
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7
Add granulated sugar, 1 tablespoon at a time, beating constantly, and beat until whites just hold stiff peaks.
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8
Beat yolks in another large bowl with cleaned beaters until slightly thickened and pale, about 5 minutes with a standing mixer or 7 with a handheld.
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9
Fold one fourth of whites into yolks to lighten.
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10
Pour yolk mixture over remaining whites, then sift flour and cornstarch on top and fold in gently but thoroughly.
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11
Fold in vanilla and zest.
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12
Spread batter evenly in baking pan and bake in middle of oven until cake is puffed and springy to the touch and top is lightly browned, about 10 minutes.
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13
Loosen paper from sides of pan.
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14
Sprinkle cake with 1/3 cup confectioners sugar, then cover with another sheet of wax paper and a baking sheet.
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15
Invert cake onto sheet and remove baking pan.
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16
Cool cake completely and peel paper from top.
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17
Spread filling evenly over cake, leaving a 1/4-inch border on all sides.
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18
Put a long platter next to a long side of cake.
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19
Beginning with a long side and using wax paper as an aid, roll up cake jelly rollstyle.
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20
Carefully transfer cake, seam side down, to platter, using wax paper to help slide cake.
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21
Chill, covered with plastic wrap, at least 2 hours.
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22
Dust cake generously with confectioners' sugar.
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23
Slice cake cold, then bring to room temperature before serving.
