Place a dry cast iron skillet (can also use stainless steel or any nonstick skillet) over medium heat and add cinnamon sticks.
Allow cinnamon sticks to toast for about 3-5 minutes, turning them occasionally with tongs to ensure even toasting. Let them toast until they're starting to darken and get fragrant.
Put water, both kinds of sugar, and cinnamon sticks in a medium-size saucepan and bring mixture to a boil.
Once boiling, lower heat and let mixture simmer for about 10 minutes, until syrup has thickened (note that it will continue to thicken as it cools).
Remove from heat and strain cinnamon sticks out of syrup into a heat-proof bowl or mason jar. Place bowl or jar in fridge to cool completely before using. Syrup can be stored in an airtight container in the fridge for up to a month.