Combine the mint leaves, cinnamon simple syrup and absinthe in the bottom of a Collins glass and gently press the mint.
Add the remaining ingredients, fill the glass with crushed ice, and swizzle to combine.
Top off the cocktail with crushed ice, and garnish with a mint sprig and a wide grapefruit peel.
*Cinnamon simple syrup: Bring 2 cups water, 2 cups sugar and 6 crushed cinnamon sticks to a boil in a small saucepan. Reduce the heat, and simmer 10 minutes. Strain and cool.