Prepare the Glass: Place the sugar cube in an Old Fashioned glass. Add the bitters and a splash of club soda (if using) to saturate the sugar cube. Use a muddler to crush the sugar to dissolve it into the bitters and soda.
Muddle the Poblano: In a cocktail shaker, muddle the diced poblano pepper with the agave syrup and lime juice. This releases the pepper's earthy flavors, creating a spicy foundation for the drink.