Fill rocks glass with ice and water to chill.
Put sugar cube in mixing glass and soak with bitters
Muddle the sugar cube into a paste
Discard ice and water and add splash of absinthe to the glass
Swirl absinthe around glass until it's thoroughly coated. Discard excess
Add ice to mixing glass and stir for 10-15 seconds.
Strain into glass
Wipe lemon peel around rim of glass and drop in cocktail.