Using a food processor or mixer, combine the flour, almond flour, sugar, salt, and lemon powder., Add the butter, and process or mix until the mixture is crumbly., Add the egg white, lemon juice (if you're using it) and grated rind, and process or mix until the dough comes together., Shape the dough into a disk, wrap it in plastic, and refrigerate for 1 hour or more., Preheat the oven to 350°F., Roll the chilled dough 1/4" thick., Using fluted cookie cutters about 2 1/2" in diameter, cut out as many cookies as possible, re-rolling the dough as needed. Note: If at any time during this process the dough becomes sticky and hard to work with, simply refrigerate it for about 20 minutes, until firm., Space the cookies 1/2" apart on lightly greased or parchment-lined baking sheets — you'll need a couple of pans., Bake the cookies for 12 to 15 minutes, until their edges begin to brown. Remove them from the oven, and transfer them to a rack to cool., For the glaze, stir together the lemon juice and confectioners' sugar until smooth., Brush a thin layer of glaze onto the surface of each cookie; garnish with sparkling sugar.