1/2 cup unsweetened Dutch-process cocoa powder, plus 2 tablespoons for icing
1 1/2 cups all-purpose flour
1/4 cup confectioners' sugar, plus more for dusting
1 1/2 tablespoons milk
Vegetable oil, cooking spray
Melt chocolate with 1 cup butter (2 sticks) in a saucepan over medium-high heat, stirring constantly. Let cool slightly.
Put eggs, vanilla, and granulated sugar in bowl of an electric mixer fitted with the paddle attachment. Mix on medium speed until pale, 4 to 5 minutes. Mix in chocolate mixture, salt, cinnamon, 1/2 cup cocoa powder, and the flour.
Heat a waffle iron until hot. Lightly coat grids with cooking spray. Spoon about 1 tablespoon batter onto center of each waffle-iron square to make 1 1/2-inch rounds. Close cover; cook until set, about 1 1/2 minutes. Transfer to a wire rack, bottom sides up. Let cool completely. Repeat with remaining batter, coating grids with cooking spray after each batch.
Melt remaining 2 tablespoons butter in a small saucepan over low heat. Add confectioners' sugar and remaining 2 tablespoons cocoa powder; stir until smooth. Stir in milk.
Gently dip 1 surface of each cookie in icing so that just the waffle lines (not gaps) are coated. Repeat with remaining cookies and icing. Transfer to wire racks; let stand until set, about 10 minutes. Dust iced surfaces of cookies with confectioners' sugar.