Pour milk and 1 cup Cinnamon Toast Crunch cereal in a bowl. Place in fridge and let rest for 1 hour so cereal flavors can infuse into milk. Strain out the cereal.
Crush remaining ¼ cup of the Cinnamon Toast Crunch (I like to put it in a Ziplock bag and whack it with a meat tenderizer) and place on a small plate.
Brush a little simple syrup, agave syrup, maple syrup, or honey on the rim of two small glasses (I use 5.5 oz. coupe glasses). Rotate glasses upside-down on plate to coat rims in cereal.
In a shaker filled with ice add 2 ½ oz cereal-infused milk, 1 ½ oz bourbon, and ½ tsp simple syrup. Shake well and pour into cereal-rimmed glass.
Repeat with second cocktail. Sprinkle ground cinnamon over each cocktail, if desired.