Heat corn syrup, butter and chocolate chips, and 5 c marshmallows to boiling in saucepan, stirring constantly.
Remove from heat.
Add vanilla.
Pour over cereal in large mixing bowl; toss quickly until well coated.
Fold in 1 c marshmallows.
Press mixture evenly in pan with piece of waxed paper.
Let stand 1 hour until firm or ridgerate if firmer bar is desired.
Cut into 2" squares. 24 squares.
Dense squares of toasted cereal and molten marshmallow yield a sticky, structurally fragile confection that demands strict adherence to sequencing. Melt the butter, corn syrup, chocolate chips, and bulk marshmallows together over low heat, stirring constantly to prevent the sugar solids from scorching against the pan bottom. Remove the saucepan immediately upon reaching a boil; lingering heat will turn the sugar brittle rather than chewy. Work rapidly once the liquid hits the cereal, because cooling syrup stiffens within ninety seconds and resists even coating. Fold the reserved mini marshmallows in gently just before transferring the mass to a greased nine-by-thirteen-inch pan, pressing the surface flat with greased hands or a sheet of waxed paper.
Temperature control
High heat destroys the emulsion of fat and sugar, separating the butter and leaving greasy pockets throughout the cereal bars. Keep the burner at medium-low when melting the binder. Whisk continuously until the chocolate and five cups of marshmallows liquefy into a glossy paste.
Storage dynamics
Refrigeration yields a firmer, cleaner cut when slicing the bars into two-inch squares, though ambient storage preserves a softer bite. Wrap individual squares tightly in parchment paper to prevent moisture absorption during room temperature storage lasting up to three days.