These chocolate-cinnamon waffle cookies were made in a waffle iron, then dusted with confectioners' sugar.
Melt chocolate and 2 sticks butter in a small saucepan over medium-high heat, stirring. Let cool slightly.
Put eggs, vanilla, and granulated sugar in the bowl of an electric mixer fitted with the paddle. Mix on medium speed until pale, 4 to 5 minutes. Mix in chocolate mixture, salt, cinnamon, 1/2 cup cocoa, and the flour.
Heat a waffle iron until hot. Lightly coat grids with cooking spray. Spoon batter onto center of each waffle-iron square (rounds should be about 1 1/2 inches). Close cover; cook until set, about 1 1/2 minutes. Remove cookies with a fork; let cool, bottom sides up, on a wire rack. Repeat (coat grids with cooking spray each time).
Melt remaining 2 tablespoons butter in a small saucepan. Add confectioners' sugar and the remaining 2 tablespoons cocoa; stir until smooth. Stir in milk.
Gently dip surface of cookies in icing so waffle lines (not gaps) are coated. Let icing set, about 10 minutes. Using a fine sieve, dust with confectioners' sugar.