For the cookies:
5 ounces (1 cup) whole hazelnuts, toasted
1 1/2 teaspoons instant espresso powder
3 ounces bittersweet or semisweet chocolate (not unsweetened), broken up or coarsely chopped
5 1/3 ounces (2/3 cup) unsalted butter, slightly softened
3/4 cup granulated sugar
1 1/8 ounces (1/3 cup) unsweetened Dutch-processed cocoa powder
1 large egg
1 1/2 teaspoons pure vanilla extract
1/4 teaspoons table salt
8 1/4 ounces (1 3/4 cups) unbleached all-purpose flour
Butter or nonstick spray for the baking sheets

For the glaze:
16 ounces bittersweet or semisweet chocolate (not unsweetened), broken up or coarsely chopped
1 tablspoon corn oil or other flavorless vegetable oil