Shake together cognac or bourbon, lemon slices, and crushed ice in a cocktail shaker. Drop a sugar cube and a curl of lemon zest into each of four flutes. Strain the cognac into the glasses and then fill each with well-chilled champagne.
If you want to dress up the drinks, you could add a few drops of Angostura bitters to each or saturate the sugar cubes with three or four drops of bitters before dropping them into the glass. You may also use orange zest instead of lemon and substitute two jiggers of Curaçao or Grand Marnier (or a jigger of each) for half of the cognac.
For a flavorful addition to champagne cocktails, use the round channel cutter on the side of a bar zester to cut a curl of lemon zest to drop into each glass.