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1
To make the cookies, position racks in the upper and lower thirds of the oven; preheat the oven to 375F (190C).
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2
Line 2 baking sheets with parchment paper or silicone baking mats.
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3
In a small bowl, whisk together the all-purpose and cake flours, baking powder, and salt.
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4
In another small bowl, mix together the milk, 1 teaspoon vanilla, and lemon zest.
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5
In a stand mixer fitted with the paddle attachment (or in a bowl by hand), beat together the butter and granulated sugar on medium speed until completely smooth.
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6
Beat in the eggs one at a time.
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7
Stir in half of the flour mixture, followed by the milk mixture, then stir in the remaining flour mixture and beat until the batter is smooth.
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8
Drop 2 tablespoons (30 ml) batter in mounds spaced 2 inches (5 cm) apart on the prepared baking sheets.
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9
Bake, rotating the baking sheets midway through baking, until the cookies feel just set in the centers, about 15 minutes.
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10
Let cool completely on the baking sheets.
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11
To make the icings, in a medium bowl, whisk together the 2 cups (225 g) powdered sugar with 2 teaspoons corn syrup, the lemon juice, the 1 teaspoon vanilla, and water until smooth.
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12
Transfer half of the mixture to a small bowl and whisk in the cocoa and remaining 2 teaspoons corn syrup to make the black icing.
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13
Add up to 2 teaspoons more water, if necessary, to make the icing spreadable; it should not be too thin, so begin by adding 1 teaspoon and add another teaspoon only if needed.
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14
Whisk the remaining 2 tablespoons powdered sugar into the white icing.
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15
The two icings should have the same consistency: thick, but spreadable.
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16
(If the white icing is too thin, add a bit more powdered sugar.)
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17
With a small icing spatula or a butter knife, spread white icing over one half of the flat (bottom) side of each cookie.
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18
Spread black icing over the other half.
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19
Let the icing set for a few minutes before serving.
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20
The finished cookies can be stored for up to 2 days in an airtight container, preferably in a single layer so that the icing wont be marred.
