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1
Make cookies: Whisk together cocoa powder and flour in a bowl.
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2
Put butter and granulated sugar in the bowl of an electric mixer fitted with the paddle attachment.
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3
Mix on medium-high speed until pale and fluffy, about 3 minutes.
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4
Mix in egg until well blended.
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5
Reduce speed to low.
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6
Add flour mixture; mix until just combined.
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7
Divide dough in half, and shape each half into a disk; wrap in plastic.
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8
Refrigerate until firm, about 1 hour (or overnight).
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9
Preheat oven to 350F.
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10
Transfer dough to a work surface lightly dusted with confectioners sugar.
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11
Roll out dough to 1/8 inch thick.
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12
Cut out cookies using a 2-inch round cookie cutter; space 1/2 inch apart on baking sheets lined with parchment paper.
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13
Repeat with remaining scraps of dough.
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14
Bake cookies until firm, rotating sheets halfway through, 10 to 12 minutes.
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15
Let cool completely on sheets on wire racks.
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16
Meanwhile, make ganache: Bring cream to a boil in a small saucepan over medium-high heat.
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17
Add chocolate.
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18
Cook, stirring constantly, until chocolate is smooth.
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19
Stir in peppermint extract.
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20
Let cool slightly, 10 to 15 minutes.
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21
Spoon 1 teaspoon ganache onto the bottom of one cookie; sandwich with another cookie.
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22
Repeat with remaining cookies and ganache.
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23
Refrigerate until firm, about 10 minutes.
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24
Make glaze: Melt chocolate in a heatproof bowl set over a pan of simmering water, stirring constantly.
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25
Let cool slightly.
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26
Dip one flat side of each sandwich into melted chocolate to coat; gently shake off excess.
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27
Place sandwiches, chocolate sides up, on wire racks set over baking sheets.
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28
Refrigerate until set, about 15 minutes.
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29
Cookies can be refrigerated in a single layer in airtight containers up to 2 days.
