Combine water and sugar in small saucepan over low heat until sugar dissolves.
Increase heat and bring to boil. Add five mint sprigs. Remove from heat; let cool completely.
Strain syrup into a large pitcher. Add grapefruit juice, bourbon and bitters and 10 mint sprigs.
Chill for several hours or overnight.
Add ice block to bowl.
Pour grapefruit and bourbon mixture over ice through a strainer.
Fill glasses with ice and fill approximately 3/4 full with bourbon and juice mixture; top with chilled soda and stir.
Garnish with mint leaves.
Notes
The recipe calls for a block of ice. A what? I can not remember the last time I saw a block of ice!So, I filled a bowl with ice cubes and poured a mixture of soda and grapefruit juice over them and put them in the freezer. By the time the juice had chilled I had my own 'ice bowl!' I put it in a large pan with warm water and within a minute or two was able to remove the ice from the bowl in one piece.Flip it over and put it in a punch bowl and there you have it!