Preheat the oven to 350 degrees . Butter a 9-by-13-inch baking pan, then line it with wax paper or parchment paper and butter the paper.
Whisk together the eggs, sugar, mayonnaise and vanilla in a large bowl. Place the cocoa and cinnamon in a small heatproof bowl; whisk in the coffee and 1/2 cup of cold water until blended. Add the coffee mixture to the egg mixture in 3 parts, whisking to blend after each addition. Add the flour and baking soda and whisk until smooth. stir in the chocolate chips.
Pour the batter into the prepared pan. Bake the cake until a toothpick inserted in the center comes out clean, about 40 minutes. Let the cake cool for 10 minutes, then invert onto a rack and let cool for about 1 hour. Dust the cake with cinnamon, then confectioners sugar, and cut into triangles.
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