Line an 8×8 square pan with parchment paper, leaving overhang on the sides
In a large saucepan or Dutch oven, add butter. Turn heat to medium and let the butter melt
Add in the marshmallows and salt
Cook on low heat until very smooth and pourable, then pour the liquid marshmallow on top of the cereal. Mix with a nonstick spatula until combined. (If you are using additional marshmallows, add these in after combining the melted marshmallow and cocoa pebbles, fold in.)
Put half of the rice krispie mixture down and lightly pat it with a nonstick spatula. Spread the hot fudge layer on top, then add the second half of the rice krispies on top of that.
Let set for at least 20 minutes before cutting in, top with some flaky salt and enjoy!