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1
Preheat the oven to 350F.
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2
Line two large baking sheets with parchment paper; set aside.
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3
In a bowl, whisk together the flour, baking powder, baking soda, and salt.
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4
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter, shortening, and sugar on medium speed until light and fluffy, about 3 minutes, scraping down the sides of the bowl as needed.
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5
Add whole eggs and egg yolk, one at a time; beat until combined after each addition.
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6
Beat in vanilla.
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7
Add the flour mixture in two batches, alternating with the cream.
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8
Beat until just combined (do not overmix).
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9
Use a 1/4-cup measure to scoop dough 3 inches apart onto prepared sheets.
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10
Bake, rotating sheets halfway through, until the edges just begin to turn golden and the centers are cakey and tender, 10 to 12 minutes.
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11
Transfer parchment and cookies to a wire rack to cool for 10 minutes.
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12
Remove cookies from parchment and let cool completely.
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13
On the flat side (bottom) of each cookie, use a small offset spatula to spread chocolate icing over one half of the cookie, creating a line straight down the center.
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14
Spread white frosting on the other half.
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15
Set cookies aside until icing is set, about 30 minutes.
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16
Whisk cream into sugar until smooth.
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17
Set aside 1 1/4 cups (for white icing).
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18
Combine cocoa with boiling water, and stir until dissolved.
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19
Add to remaining sugar mixture; stir to combine (for black icing).
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20
Use immediately.
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21
One half of the flat side of each cookie is carefully covered with chocolate icing using a small offset spatula; white icing is spread on the other half.
