Adapted from Gourmet
2 cups flour
1/2 cup Dutch process cocoa powder (I used Droste)
1/2 teaspoon baking powder
1/2 teaspoon salt
2 sticks butter, at room temperature
3/4 cup sugar
1 egg yolk
1 teaspoon vanilla
1/2 cup heavy cream
1 1/2 tablespoons light corn syrup
3/4 pound white chocolate, melted
2 tablespoons butter, at room temperature
2 teaspoons vanilla
Whisk together flour, cocoa, baking powder and salt. Beat butter and sugar with electric mixer till light and fluffy, then add yolk and vanilla. Slowly add flour, mixing in on low speed. Divide dough in half, wrap each half in plastic wrap and refrigerate till firm...about 2-3 hours.
Make ganache by heating cream and corn syrup in saucepan. Add melted chocolate and stir to combine. Add butter and vanilla and mix till smooth. Cover with plastic and refrigerate, stirring occasionally, till thick...about a half hour.
Preheat oven to 350º. Line two baking sheets with parchment.
Roll out one square of dough between two sheets of parchment to a thickness of 1/8 inch. Freeze flattened dough for 10 minutes. Cut out rounds with a 1 3/4 fluted cutter and arrange 1/2 inch apart on baking sheets. Bake about 10 minutes. Cool on baking sheet for 5 minutes then remove to cooling rack to finish cooling. Reuse scraps of dough to form more cookies...putting dough in freezer to chill if it gets too soft.
To assemble cookies, beat ganache with electric mixer till light and fluffy. Put ganache in ziplock bag and seal. Cut corner off bag and pipe ganache onto flat side of one cookie. Top with second cookie to form a sandwich. Chill cookies till ganache is set.