In a large pot, heat the butter, marshmallows, maple syrup, cinnamon, and salt over low heat until the marshmallows start to melt, about 5 minutes. Whisk every now and then to incorporate the ingredients.
Once you have a homogeneous mixture, turn off the heat and add the vanilla. Stir to combine.
Then, add the cornflakes. Using a nonstick spatula, fold the ingredients so that the marshmallow mixture coats the cereal.
Line a baking sheet with parchment paper or plastic wrap. Dump the crispy treat mixture onto the lined baking sheet. Using another piece of plastic wrap, press the mixture firmly to even out the bars.
Allow to cool completely in the fridge for 2-4 hours.
Once cooled, cut the bars into bite-sized pieces. Serve and enjoy!