Preheat oven to 350°. With 3-in. leaf-shaped cookie cutters, cut out as many cookies as you can (reroll scraps to make more cookies), setting them on a parchment-lined baking sheet as you go. Prick cookies all over with a fork (this will keep them from puffing up as they bake).
With fork, mix egg with 1 tbsp. water to make an egg wash. Brush cookies with egg wash, then stir cardamom and sugar together and sprinkle it onto cookies.
Bake cookies, rotating pan position halfway through, until golden brown, 10 to 12 minutes. Sprinkle warm cookies with more cardamom sugar and let cool completely on baking sheets.