Adapted from a King Arthur Flour recipe. The most authentic version calls for a walnut -shaped cookie mold such as this, available from various online baking supply outlets.
1/2 cup butter, room temperature
1/3 cup dark brown sugar
1/4 teaspoon salt
1 teaspoon vanilla
1 cup flour
2 tablespoons cornstarch
1/2 cup finely ground walnuts
2 tablespoons heavy cream
1/4 cup bittersweet chocolate chips
Preheat oven to 350 degrees. Cream butter, brown sugar, salt and vanilla. Stir together flour and cornstarch. Beat into butter mixture along with the ground walnuts. Refrigerate the dough for at least 30 minutes.
Press a scant teaspoonful of dough into the mold, pressing into the bottom and up the sides–leaving a small indentation in the center. Do not fill completely. Bake for 9 to 10 minutes, until cookies are browned around the edge and golden in the center. Cool for a few minutes, then pop the cookies out of the molds with the tip of your fingernail or a knife, working from the thick edge of the cookie. Cool completely.
To make the ganache filling, place the chocolate in a small, microwave-safe bowl and stir in the cream. Gently heat (about 1 minute) until the chocolate just begins to melt. Remove from microwave and whisk until smooth. Allow to cool slightly, then spread a bit of the ganache on one cookie and sandwich with another half, repeating until you have 2 dozen walnut-shaped cookies.
Variation: Melt 1/4 cup of apricot or raspberry jam and spread between the two halves.