The French 75 is a historic cocktail which, strangely enough, can be made with gin and Champagne, or Cognac and Champagne. It was invented either during World War I or immediately thereafter, and it is named after the French 75mm field gun. As far as I can tell, the combination of gin and Champagne predates the Cognac and Champagne version that is perhaps more familiar today. (I intend to examine the question more closely, but as the 1930 Savoy cocktail book gives the recipe as involving gin and Champagne with no variations, I am inclined to believe that the Cognac is a later addition.)
That said, I will now furnish you with the French 75 recipe with Cognac, because it is well worth sharing:
1 ounce Cognac
a teaspoon of sugar
1/2 ounce fresh lemon juice
Champagne to fill the glass
Combine the Cognac, sugar, and lemon juice in a cocktail shaker and shake vigorously. Strain into a well-chilled Champagne glass, and fill to the top with Champagne.
More on other variations later, especially the classic gin and Champagne version. Also, in the near future: Corpse Revivers.