from Death & Co
In a shaker, gently muddle the mint with the simple syrup and sugar cube. Add the rum and lime juice and whip (shake with a few pieces of crushed ice just until the ingredients are incorporated). Dump into a pilsner glass filled with crushed ice. Swizzle until cold, then top with the bitters and swizzle them into the top of the drink. Garnish with the mint sprig and serve with a straw.