Cognac is brandy that comes from Cognac, France, and meets specific criteria: it has to have been aged in oak casks for at least two years at a storage facility that is registered with the Bureau National Interprofessionnel du Cognac (BNIC); the minimum level of alcohol content when sold must be 40%; no additives may be put in the cognac; and a label on the bottle should indicate all information about the spirit, exactly where the grapes were harvested from as well as the name and the address of the cognac producer.
Brandy is a term introduced in Europe in the 16th century. It comes from the word brandewijn, which means "burnt wine." The Dutch found some of their barrels to be awful when considered as wine, but fine for a new spirit. Since cognac is brandy that just doesn't meet the specific criteria to have the same title, you would have no problem substituting brandy for cognac when cooking. Just check the label to make sure it's not a flavored brandy, such as apricot, if you don't want to alter the taste.
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Cognac is made of grapes just as wine is. Try substituting wine in your recipes that ask for cognac. Be cautious about the type of wine you use as you want it to taste similar to cognac. Look for a medium-bodied to full-bodied wine that isn't too sweet, like a sherry or a port wine. Avoid bold wines with pepper tones such as syrah (called shiraz in Australia) as it may change the flavor of your finished dish.
You can also substitute cognac in a recipe with a non-alcoholic beverage. Cognac is made from grapes so that is the obvious choice, but make sure you are using unsweetened grape juice. You can try other juices such as apple, peach or apricot as long as they have no added sugar. In addition, non-alcoholic wine and brandy are available in the supermarket if you want to stick to a taste that is closer to brandy.
When you have time and aren't in a rush to prepare your recipe, consider experimenting with different flavored liquids. You could create some interesting variety in the taste of your finished product. Try a chicken with apple brandy instead of cognac, or a cake with pineapple juice. Begin with the same proportion of liquid that is in your recipe for cognac and then adjust if the finished product is to loose or dry.