Muddle the plums, lime juice and simple syrup (if using) in the bottom of a glass.
Add ice, whiskey and ginger beer. Give it a stir.
Garnish with a plum wedge + slice of lime.
Notes
Whiskeys vary in sweetness. Likewise, ginger beers vary in levels of sweet and spice. If you like your cocktail a little sweeter, you may add simple syrup or super fine sugar to the muddling step.