Line an 8x8-inch baking pan with parchment paper. Then, place the prepared pan and your chocolate chips in the freezer for at least an hour to chill.
Melt the butter in a large pot over medium heat. Add marshmallows and stir until melted and smooth. Add vanilla and stir to combine.
Remove from heat and stir in cereal, millet and a pinch of flaky sea salt. This will be messy!
Working quickly, add the frozen chocolate chips and stir just until combined. Immediately transfer the mix to the prepared frozen baking pan. Cover the mixture with a piece of parchment paper and press with your hands or the base of a sturdy glass cup to flatten the treats. Immediately return the pan to the freezer to chill. This step should take no longer than 2 minutes - speed is important to keep the chocolate from melting!
After the treats have fully chilled for 1 hour, turn them onto a cutting board and cut into 9 squares. Serve immediately.