Cook over medium heat until the water just starts to boil. Add in 1 cup of granulated sugar at this point and stir continuously until the sugar is fully dissolved. This should only take about 15 seconds! The mixture should look clear.
Remove from heat and stir in 1 tsp of peppermint extract. Waiting to add the extract until you've removed that pan from heat will prevent it from evaporating and losing some of its flavor. I find 1 tsp of peppermint extract is the perfect amount for my palette, but if you like a less strong peppermint flavor I recommend adding 1/2 tsp.
Pour into a separate container and cool to room temperature. I like to keep my simple syrup in glass containers or mason jars for easy storage.
If you're making this in advance, it can be stored in an airtight container in the fridge for up to a month.
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Notes
This recipe makes about 10 oz. or 1 1/4 cups of peppermint simple syrup. This recipe can be halved, quartered, or doubled if need.Peppermint Exact - Any peppermint extract will work, but I recommend using good quality extract because it has a huge impact of the flavor of this syrup. Make sure you don't use peppermint oil or peppermint "flavor."