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1
In a large bowl, sift together the flour, cocoa, baking powder, and salt.
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2
Set aside.
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3
Cream the butter and sugar until soft and creamy in the bowl of an electric mixer fitted with the paddle attachment.
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4
Beat in the egg and vanilla.
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5
Add the flour mixture, and mix on low speed until combined.
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6
Divide the dough in half; wrap each half in plastic wrap.
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7
Chill at least 1 hour.
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8
Preheat the oven to 325F.
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9
On a lightly floured surface, roll the dough to 1/8 inch thick.
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10
Cut into 1 1/2-inch squares using a cookie cutter.
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11
Transfer to parchment-paperlined baking sheets, and refrigerate until firm, about 30 minutes.
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12
Gather any scraps, wrap them in plastic wrap, and chill for 30 minutes before rerolling and cutting into squares.
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13
Bake until the edges just begin to brown, about 12 minutes.
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14
Transfer to wire racks to cool.
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15
To make the white chocolate ganache, bring the cream to a boil in a small saucepan.
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16
Remove from heat, and add 9 ounces white chocolate.
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17
Let stand for 5 minutes, covered.
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18
Transfer the mixture to a medium bowl, and stir until completely smooth.
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19
Place in the refrigerator, and chill until thick enough to spread, about 1 hour, stirring every 10 minutes.
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20
Use an offset spatula to spread the ganache on half the cookies; top with the remaining cookies.
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21
Melt the remaining 3 ounces white chocolate in a small heatproof bowl set over a pan of simmering water.
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22
Make a small cone out of parchment paper, or fit a small pastry bag with a coupler and #2 Ateco tip.
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23
Fill, and decorate as desired.
