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In 1880s France, oranges were quite rare and exotic. When Louis Alexandre Marnier-Lopostolle traveled to the Caribbean in search of ingredients, he came back with bitter oranges to combine with his family's fine cognac. Other orange-flavored liqueurs such as triple sec and curacao are mixed with a neutral alcohol base. Grand Marnier took it to the next level with a more complex flavor that makes it today's top-selling French liqueur.
Now you too can combine cognac with a real orange to make a home version of the tasty—and pricey—stuff. By using an inexpensive cognac that costs around 18 to 20 dollars a bottle, you can create a clone cousin of the real thing that normally sells for around 30 bucks a bottle. All you need, in addition to the cognac, is some sugar, an orange, and a little patience to wait at least 2 weeks.
Source: Top Secret Recipes: Sodas, Smoothies, Spirits & Shakes by Todd Wilbur.