Preheat oven to 375 degrees. In a small pan over low heat melt the butter and chocolate together. Meanwhile, mix the flour, baking powder, baking soda, and salt together. Cool the chocolate and pour into the bowl of a mixer. Beat in both sugars, the vanilla, sour cream or buttermilk, and egg. Add the flour mixture, nuts, and chocolate chips and mix well. Grease cookie sheets. For 3-inch round cookies, drop the batter by teaspoonfuls onto the cookie sheet, leaving about 2 inches in between each cookie. Bake at 375 degrees for 8 to 10 minutes. Do not let edges brown. For 42-inch round cookies, drop the batter by tablespoons, leaving about 2 inches between each cookie. Baking time will be 12 to 15 minutes. Use a spatula to transfer the cookies to racks to cool. To make the ganache filling, melt the chocolate and butter in a small pan over low heat or in the microwave. Gradually whisk in the cream. Let stand at room temperature, stirring occasionally, for 1/2 hour or longer, until barely thickened. Or place the bowl in ice water if you are in a hurry, but don't let the filling harden. Turn half the cookies upside down on a large piece of wax paper. Spread a tablespoonful on each of the turned cookies. Quickly cover with the remaining cookies, right sides up, and press gently down to spread the filling to the edges. Let stand at room temperature or chill briefly until the filling is set.

Makes 20 small or 9 large sandwiches