Prep and Cook Time: about 3 3/4 hours. Notes: To melt the chocolate, place in a heatproof bowl set over a pan of barely simmering water (bottom of bowl should not touch water); stir often until melted and smooth, about 5 minutes. These bite-size sandwich cookies are best eaten the day they are filled, but you can make the cookies and filling up to 2 days ahead. Store the baked, unfilled cookies airtight at room temperature; cover and chill the vanilla filling.
1 cup (1/2 lb.) butter, at room temperature
1/2 cup sugar
1 large egg yolk
1 teaspoon vanilla
2 1/2 cups all-purpose flour
2 ounces unsweetened chocolate, melted (see notes)
Vanilla cream filling (recipe follows)
How to Make It
In a bowl, with an electric mixer on medium speed, beat butter and sugar until smooth. Beat in egg yolk and vanilla, scraping down sides of bowl as needed. Stir or beat in flour until well blended.
Divide dough in half and mix melted chocolate into one half. Divide each color of dough into four equal portions. Form each portion into a 3/4-inch-thick log. Roll waxed paper tightly around each log to wrap airtight. Freeze until firm enough to slice neatly, about 20 minutes.
Unwrap dough. Using a thin, sharp knife, cut logs crosswise into 1/4-inch-thick slices (if chilled dough crumbles, let stand a few minutes to soften, then cut). Lay slices 1 inch apart on buttered or cooking parchment-lined 12- by 15-inch baking sheets; if cookies flattened when cut, reshape with your fingers.
Bake in a 350° oven until plain cookies are lightly browned on the bottom and chocolate ones are slightly darker, 8 to 10 minutes; if baking more than one pan at a time, switch pan positions halfway through baking. With a wide spatula, transfer cookies to racks to cool completely.
Spread about 1/4 teaspoon vanilla cream filling onto each plain cookie, then top with a chocolate cookie.
Vanilla cream filling. In a bowl, with an electric mixer on medium speed, beat 1/2 cup (1/4 lb.) butter (at room temperature), 3/4 cup powdered sugar, and 1 teaspoon vanilla until smooth. Use immediately, or cover and chill for up to 2 days (bring to room temperature and beat to soften before using).
Note: Nutritional analysis is per sandwich cookie.