Lightly beat the reserved egg white. Place hazelnuts in a small bowl. Shape dough into 1-inch balls using a small cookie scoop
. Roll balls in egg white, then in hazelnuts to coat. Place balls 1 inch apart on the prepared cookie sheet. Press you thumb into the center of each ball. Bake for about 10 minutes, or until the edges are firm. If cookie centers puff during baking, re-press with the back of a spoon.