Moss Sandwich Cookies
In a medium bowl, whisk together flour, cornstarch, and salt. In a stand mixer bowl fitted with paddle attachment, beat butter with sugar on medium speed until light and fluffy, 2 minutes. Beat in vanilla. Reduce speed to low and gradually add flour mixture; beat until combined. Tint dough with food coloring until deep green. Divide dough in half; shape each piece into a disk. Cover tightly with plastic wrap and refrigerate until firm, at least 1 hour.Advertisement
Preheat oven to 350°F, with racks in upper and lower thirds. Line 2 baking sheets with parchment. Working with one disk at a time, scoop teaspoons of dough, roll into balls, and place on baking sheets, about 1 inch apart. Flatten cookies slightly with a spatula.
Bake, rotating sheets halfway through, until edges are just set, 10 to 12 minutes. Let cool 5 minutes on sheets, then transfer to a wire rack and let cool completely. (Baked cookies can be stored in an airtight container at room temperature up to 3 days.)
Crush 6 cookies until finely ground. With an offset spatula, spread 1/2 teaspoon dulce de leche each on flat sides of half of remaining cookies; sandwich with other half. Refrigerate until firm, 30 minutes. Sprinkle tops with crushed cookie "moss." (Filled cookies are best eaten the same day.)