Coat the inside of the rocks glass with a splash of absinthe (or herbsaint).

In a glass pitcher, combine the Keto simple syrup, Peychaud's bitters, and the lemon peel. Muddle it lightly in the glass to express the oils in the lemon.

Toss in a few pieces of cracked ice and add in the cognac and the rye whiskey. Stir until chilled.

Strain into the rocks glass and serve (or don't serve) over a large single ice cube.