Preheat oven to 350 degrees. On parchment-lined baking sheets, drop all-in-one cookie dough by heaping tablespoons, 1 inch apart.
In a small bowl, whisk 1 large egg white with 1 teaspoon water. Brush cookies with egg wash. Sprinkle with desired topping (makes about 28). After forming cookies, bake until pale golden, 13 to 15 minutes, rotating baking sheets halfway through.
Thumbprint Variation After step 1, use a 1/4-teaspoon measure to gently press a hollow in the center of each dough mound. Fill each hollow with 1/2 teaspoon jam or jelly, a chocolate chunk, or a nut half (makes about 28).
Sandwich Variation After step 1, use bottom of a floured glass to flatten mounds into 2-inch rounds. With a 1-inch cookie cutter, cut out center from half the rounds. Decrease baking time by 3 to 5 minutes (makes about 14).Assemble: Using an offset spatula or a table knife, spread half the baked sandwich cookies (those without cutouts) with chocolate or fruit filling. Top with cut-out cookies; dust tops with confectioners' sugar. Let stand until set.Chocolate Filling In a microwave-safe bowl, microwave a 4-ounce bar chopped, in 20-second increments, stirring, until melted.Fruit Filling Using 2 teaspoons per cookie, stir until smooth.