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1
Put butter, whole eggs and yolks, granulated sugar, salt, vanilla, zests, Cognac, and sour cream in the bowl of an electric mixer fitted with the whisk attachment; beat on medium speed until pale and thick, 8 to 10 minutes.
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2
Reduce speed to low; gradually add enough flour to form a fairly stiff dough.
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3
Turn out dough onto a floured work surface; knead until dough blisters, becomes elastic, and can be handled easily, 6 to 8 minutes, adding flour if needed.
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4
Divide dough into four pieces.
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5
Keep dough under an inverted bowl to prevent it from drying out.
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6
Working with one piece at a time, roll out dough to 1/8 inch thick.
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7
If dough becomes too elastic while rolling, cover with plastic and let rest 15 minutes.
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8
Cut out leaves with a leaf-shape cookie cutter; transfer to a tray lined with parchment paper.
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9
Repeat with remaining dough, layering leaves between sheets of parchment.
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10
Collect all scraps, and let rest 20 minutes before rerolling.
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11
Heat oil in a medium to heavy saucepan until it registers 375F on a deep-fry thermometer.
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12
Stretch leaves slightly so they will curl while frying.
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13
Fry in batches of 12, turning occasionally, until pale golden brown, about 3 minutes.
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14
Transfer to baking sheets lined with paper towels to drain and cool.
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15
Cookies can be stored in airtight containers at room temperature up to 2 days.
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16
Sprinkle with confectioners sugar before serving.
