In a bowl, using a handheld electric mixer, beat the butter with the brown sugar at medium-high speed until fluffy, 4 minutes. Beat in the egg, egg white and vanilla until combined. Sift the flour with the cocoa and baking soda to remove the cocoa lumps. Add the mixture to the bowl along with the pepper and beat at medium-low speed until the batter is moistened. Beat in the buttermilk; the batter will be stiff.
Preheat a waffle iron and spray with vegetable oil. For each cookie, spoon 2 tablespoons of batter into a quadrant of the waffle iron; if the iron is large enough, there may be room to make more than one cookie at a time. Close and cook just until fragrant, about 2 minutes, depending on your machine. Carefully lift the cookie and transfer it to a wire rack. Dust the cookie with confectioners' sugar and repeat with the remaining batter, spraying the waffle iron as needed. Serve the cookies warm or at room temperature.