The best sandwich cookies are soft enough to bite through without the filling squeezing out. To achieve this, slightly underbake the cookies. The cookies will also soften after they are refrigerated.
Sprinkle the cookies with the sanding sugar, if using, and bake according to the Chocolate Sugar Cookies recipe for 12—15 minutes. Let cool completely, about 30 minutes.
Spoon the filling into a paper cone or pastry bag fitted with a ¼-inch (6-mm) plain round tip. Turn half of the cooki