In a freezer-safe resealable container, stir together whiskey, Cognac, superfine sugar, and bitters until sugar is dissolved. Cover and freeze for at least 8 hours or overnight. (Mixture will not freeze.)
In the container of a blender, place whiskey mixture and ice cubes. Cover and blend until smooth.
Fill 2 old-fashioned glasses with ice, and let stand until very cold, about 5 minutes.
Empty ice-filled glasses, and add ¼ teaspoon absinthe to each. Swirl to coat sides, and pour out excess. Pour chilled whiskey mixture into absinthe coated glasses. Garnish with orange twists, if desired.
Recipe by Louisiana Cookin' at https://louisianacookin.com/sazerac-slush/