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I was surprised when I learned about the mayo thing. Since it was added to the mixture of egg, butter, and the sugar, the mayo was only adding extra smoothness to the batter. There was no aftertaste of mayo at all once the chocolate and the flour were added to the mixture. Also, another secret is the stout. Instead of water, it is much better using stout to dissolve the instant coffee. But I didn't have that at home, so I was still using water.