3/4 fluid ounce lime juice
9 fresh mint leaves, divided
2 fluid ounces dark spiced rum
1 1/2 fluid ounces simple syrup
2 dashes bitters, or more to taste
1 slice lime
2 fluid ounces Champagne, or more to taste
Gently muddle lime juice with 8 mint leaves in a shaker using a wooden spoon or cocktail muddler.
Add rum, simple syrup, bitters, and a handful of ice. Shake until the outside of the shaker has frosted. Strain into a cocktail glass and top with Champagne. Garnish with a slice of lime and the remaining mint leaf.