Line a 9-x9-inch baking pan with parchment paper, leaving a 2-inch overhang on 2 opposite sides; set aside.

In a parchment-lined rimmed baking sheet, evenly spread 4 cups marshmallows. Using a blowtorch, toast marshmallows, keeping torch 2 inches above marshmallows and slowly sweeping flame across the surface until lightly charred on top (It's OK if some catch on fire— just blow them out). Set aside. (If you do not have a blowtorch, see notes below.)

In a large saucepan, melt butter over medium-low heat. Cook, stirring occasionally, until butter is lightly browned and has a nutty aroma, 4 to 8 minutes. Stir in 4 cups untoasted marshmallows and reserved toasted marshmallows and cook, stirring constantly until completely melted, 6 to 8 minutes. Remove from heat and stir in vanilla and kosher salt. Immediately transfer butter-marshmallow mixture to a heatproof bowl.

Quickly add cereal to butter-marshmallow mixture in bowl. Using a flexible rubber spatula, fold until well-combined. Fold in chocolate and 2 cups untoasted marshmallows.

Quickly transfer cereal mixture to prepared baking pan, and using a spatula and wet hands, press into an even, compact layer. Sprinkle evenly with remaining ½ cup marshmallows. Set aside to cool to room temperature, about 2 hours.

Using a blowtorch, brown sprinkled marshmallows, keeping torch 2 inches above marshmallows and slowly sweeping flame across them until lightly charred on top. Sprinkle evenly with flaky sea salt.

Remove treats from pan using parchment paper as handles. Transfer to a cutting board. Using a sharp serrated knife, cut treats into 16 equal squares. Serve.