First, lightly grease a 9x9" baking dish with spray or butter and line with parchment paper for easy removal. This will yield thick squares, but if you prefer thinner rice krispie treats, use a 9x13" baking dish instead. Set aside.
Add sliced butter to a large pot and bring it up to medium-low heat. Once melted, start stirring constantly until the milk solids at the bottom turn golden brown and you can smell a nutty aroma — this usually takes about 5 minutes.
10 tbsp salted butter
Reduce heat to low and add in maple syrup. Stir and let the maple syrup reduce for 1 more minute. Then, add in the mini marshmallows and stir until completely smooth and melted.
3-4 tbsp maple syrup, 10 oz mini marshmallows
Remove from heat and stir in the vanilla, cinnamon & kosher salt. Stir in the crispy rice cereal until evenly coated, then spread out into the prepared baking dish. Use greased hands, or a spatula, to gently level out and compress the rice krispie treats. Optionally, top with a sprinkle of flaky sea salt. Let cool at room temperature for about 1 hour before slicing into 12 squares.
1 tsp vanilla extract, ½ tsp cinnamon, ½ tsp kosher salt, 6 cups crispy rice cereal, flaky sea salt