Line with parchment paper or grease 2 large baking sheets. (Macaroons spread more on a greased baking sheet.)
In a food processor fitted with a metal blade, process together almonds and sugar until finely ground. Add egg whites and flavored water or almond extract and process until mixture forms a paste. Refrigerate until firm enough to shape (at least 30 minutes).
With moistened hands, form about 2 tablespoons of nut mixture into 1" balls and place on prepared trays, leaving 1-1/2" between cookies. (Unchilled mixture can be piped from a pastry bag fitted with 3/4-" tip.) Press a whole almond into center of each cookie.
Bake, switching baking sheets halfway through, until lightly browned (About 20 minutes). If desired, sprinkle with confectioners' sugar. Let cool on parchment paper or transfer to a rack to cool. (Store in an airtight container at room temperature for 1 week or in the freezer for up to 1 month.)
