Special equipment: a candy thermometer; a rubber mallet; a chisel, optional
Line an 8-inch square baking pan with parchment, leaving a 1-inch overhang. Mix together the egg white, confectioners' sugar and cocoa in a bowl with a fork until smooth and very thick. (The mixture should hang on the fork in a blob without falling off.) Whisk in the vanilla and vinegar. Whisk in enough food coloring to turn the mixture a deep black color and the brown of the cocoa is no longer visible, 1/2 to 1 teaspoon.
Combine the granulated sugar and 1/2 cup water in a small saucepan and stir until the mixture looks like wet sand. Using a wet pastry brush, wipe down the sides of the pot where any sugar has clung to the side. Cook over medium heat until the mixture registers 258 to 260 degrees F on a candy thermometer, 8 to 10 minutes. Remove the pan from the heat and immediately stir in the black mixture, stirring vigorously until smooth. Spread in the prepared pan and let harden and cool completely, about 2 hours.
Lift the candy out of the pan using the parchment paper overhang as handles. Peel off the parchment and chip into small irregular-sized pieces using a chisel or a sturdy butter knife and rubber mallet.
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