I make the dough a day ahead of time, and roll it out in two batches. Each half of the dough is rolled between two pieces of parchment paper. Then I place them on top of an inverted cookie sheet, and place it in the fridge. (The cookie sheet allows me to easily remove the thin sheets of cookie dough without fear of destroying the dough while transporting it from the fridge to the kitchen counter.) One more important tip about the cookies: Do NOT store them in an airtight container because the cookies will become soggy and soft. I would suggest placing them in a traditional cookie jar or cake platter with a domed top in order to retain a crisp cookie.
Without further delay, here's the recipe for Double-Chocolate Sandwich Cookies (from Gourmet):
For the Dough:
2 cups all-purpose flour
1/2 cup unsweetened Dutch-process cocoa powder
1/2 teaspoon baking powder
1/2 teaspoon salt
2 sticks (1/2 pound) unsalted butter, softened
3/4 cup sugar
1 large egg yolk
1 teaspoon pure vanilla extract
For the Ganache:
1/2 cup heavy cream
1 1/2 tablespoons light corn syrup (I use golden syrup because I don't like corn syrup. Golden syrup is available from whole foods.)
3/4 pound fine-quality white chocolate, melted
2 tablespoons unsalted butter, softened
2 teaspoons pure vanilla extract
For the Dough:
Whisk together the dry ingredients (not including the sugar).
Beat butter and sugar with an electric mixer until pale and fluffy, then beat in yolk and vanilla. At low speed, mix in flour mixture in 3 batches just until a dough forms. Divide dough in half and form into a disk, then chill, wrapped in plastic wrap, until firm, 2 to 3 hours.
While the dough chills, make the ganache:Bring cream and corn syrup just to a simmer in a small heavy saucepan, then stir into melted chocolate. Stir in butter and vanilla until smooth. Cover surface with cling wrap and chill, stirring occasionally, until very thick, about 30 minutes.
Back to the Cookies:
Preheat oven to 350°F with rack in middle. Line two 2 large baking sheets with a silicone liner or parchment paper.
Roll out 1 piece of dough between sheets of parchment paper about 1/8 inch thick. Slide dough in parchment onto a tray and freeze until dough is firm, about 10 minutes. Repeat with remaining dough.
Cut out as many rounds as possible with a 1 3/4 inch round cookie cutter, reserving and chilling scraps, then quickly transfer cookies to a buttered baking sheet, arranging them 1/2 inch apart.
Bake cookies until baked through and slightly puffed, 10 to 12 minutes. Cool on sheet on rack 5 minutes, then transfer to rack to cool completely (cookies will crisp as they cool).
Beat ganache with an electric mixer at high speed just until light and fluffy. Spread ganache onto flat sides of plain cookies, then top with another cookie to make sandwiches. Chill, layered between sheets of parchment, in an airtight container until filling is set, at least 1 hour.
Makes about 3 to 3 1/2 dozen cookies.