In a large saucepan, melt butter over low heat. Add marshmallows (reserving a handful for the top) and cook and stir until smooth. Remove from heat. Stir in the Chocolate Chex cereal until well coated.
Fold in the reserved marshmallows, before transferring into a pan. Using a buttered spatula, press firmly into a greased 13x9. pan and sprinkle the chocolate and mint chips on top and the flaky sea salt. Cool completely. Cut into bars. Serve immediately or store in an airtight container.
Notes
I used mint chips for St Patrick's Day but you can swap that out or use vanilla Chex, etc. to make your own take.