The syrup may seem like a bit of work, but it's very easy and well worth the time. Make a few batches and store it in ice cubes trays so you can soak up some summer refreshment all year long.
For the cantaloupe syrup
1 cup cantaloupe, cut into chunks
1/2 cup/100 g sugar
For the cocktail
1 1/2 ounces cantaloupe syrup
1 ounce fresh squeezed lime juice
1 ounce vodka
1 mint leaf, plus another for garnish (optional)
freshly opened bottle of seltzer, to top off the glass
To make the cantaloupe syrup, place the chunks of fruit into the bowl of a food processor. Pulse to coarsely chop, then let machine process until it becomes a smooth puree, about 1 to 2 minutes. Pour into a saucepan and stir in the sugar. Bring to a boil and cook for 1 minute more, until all the sugar has dissolved. Skim foam from top, set aside and let cool.
Strain cooled cantaloupe syrup to remove foam and "sediment". You should be left with 1/2 cup of clear, thin syrup. Store in a covered container and store it in the refrigerator to chill.
Fill a cocktail shaker with ice. Add all the cocktail ingredients, and with the top on, shake until beads of sweat form on the outside. Fill a highball glass with ice. Strain the drink into the glass. Top off with seltzer, about 2 ounces worth. Garnish with mint leaf, if desired.