Set a heavy-bottomed saucepan over medium heat and add 1 cup (200 g) granulated sugar in an even layer. Using a heatproof spatula or wooden spoon, stir constantly as the sugar melts — it will clump, then liquefy and turn amber. Be patient and keep the sugar moving so it doesn’t burn.

When the sugar reaches a medium-amber color and smells toasty, reduce heat slightly and add the 6 tablespoons (85 g) butter pieces. The mixture will bubble vigorously — stir until butter is fully incorporated.

Remove pan from heat briefly and slowly pour in the warm heavy cream (120 ml), stirring continually (it will steam and bubble). Return to low heat and simmer 30–60 seconds until silky and slightly thickened.

Stir in vanilla (if using) and ½–¾ teaspoon flaky sea salt. Taste and adjust for balance. Transfer caramel to a heatproof bowl and let cool until still-pourable but not piping hot.

Line a 9×13-inch pan with parchment paper (leave an overhang) or grease lightly. This ensures easy removal.

In a large pot over low–medium heat, melt 4 tablespoons (56 g) butter. Add the marshmallows and stir constantly until they melt into a glossy, smooth mass. Remove from heat as soon as melted to avoid graininess.

Stir in ½ teaspoon vanilla extract and ¼ teaspoon fine sea salt.

Add the rice cereal to the melted marshmallow in two batches, folding gently but thoroughly so each flake is coated. Reserve about ½ cup cereal if you’d like extra texture for topping.

Spread about two-thirds of the cereal-marshmallow mixture into the prepared pan to form an even base layer.

Spoon ribbons or dollops of the warm (but not scalding) salted caramel over the layer. Use a spoon or small piping bag to place caramel in lines or dollops for swirling. If using chocolate, drizzle melted chocolate over the caramel now or after the top layer is added.

Gently top with the remaining cereal-marshmallow mixture and press lightly with an oiled spatula or parchment to achieve an even surface — be careful not to over-press (aim to compact, not squash).

Immediately sprinkle a light pinch of flaky sea salt over the top while caramel/chocolate are still tacky.

Let cool at room temperature for 30–45 minutes to set. For cleaner slices, refrigerate 20–30 minutes before cutting, but avoid chilling so long that bars become rock-hard.

Using the parchment overhang, lift the slab from the pan and cut into 12 large bars or 20–24 snack squares with a sharp knife (wipe blade between cuts for neat edges).