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1
Whisk the flour, 1/4 cup cocoa powder, the baking powder and salt in a bowl.
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2
Beat 1 stick butter and the granulated sugar in a large bowl with a mixer on medium speed until light and fluffy, 3 to 5 minutes.
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3
Beat in the egg and 1/2 teaspoon vanilla.
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4
Reduce the mixer speed to medium low; beat in the flour mixture until just incorporated.
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5
Divide the dough between 2 sheets of plastic wrap; form into flat rectangles.
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6
Wrap and refrigerate until firm, at least 1 hour.
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7
Position racks in the upper and lower thirds of the oven; preheat to 350 degrees F. Line 2 baking sheets with parchment.
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8
Working with 1 piece at a time, roll out the dough into a 10-inch square (1/8 inch thick) on a lightly floured surface.
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9
Cut into 1-by-2 1/2-inch rectangles.
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10
Transfer to the prepared baking sheets; prick with a fork and sprinkle with granulated sugar.
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11
Chill until firm, 15 minutes.
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12
Bake, switching the pans halfway through, until the cookies are firm, 13 to 16 minutes.
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13
Let cool completely on the baking sheets.
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14
Put the chocolate in a microwave-safe bowl; microwave in 30-second intervals, stirring, until smooth; let cool.
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15
Add the remaining 1/2 stick butter; beat with a rubber spatula until incorporated.
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16
Mix in the remaining 1 tablespoon cocoa powder, the confectioners' sugar and the remaining 1/2 teaspoon vanilla until smooth.
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17
Sandwich between the cookies.
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18
Photograph by Levi Brown
