Old Fashioned with Crème de Cacao in place of sugar. Source 3-Ingredient Happy Hour: https://skillet.lifehacker.com/why-your-old-fashioned-needs-creme-de-cacao-1838780532
(Try it with Orange Bitters instead)
2 ounces bourbon
1/2 ounce crème de cacao
3 dashes of peach bitters (try with orange Bitters)
Add everything to a lowball with a great big cube and give it a little stir. Garnish with a strip of lemon zest if you need a garnish (and are okay with upping your ingredient count).