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