From Death & Co
Garnish: 1 mint sprig
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, then top with the bitters and swizzle them into the top of the drink.
Garnish with the mint sprig and serve with a straw.