what can one use instead of cognac in the chocolate mousse cake?
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Rum, Brandy, Grand Marnier if you like chocolate/orange.
I have used whiskey, bourbon, and cognac as substitutes for each other in various desserts. Maybe my taste is not sensitive enough, but most recipes I've encountered only call for a few tablespoons of the alcohol in question and I can't taste the difference. If you're looking for a non-alcohol substitute, espresso or strong coffee adds even more depth of flavor to chocolate desserts.
Oh and brandy is another one. In my opinion, chocolate and orange go well with a number of brown liquors, so it's pretty forgiving to swap them.