Line a 9x9 inch pan with parchment paper and grease lightly.

For the base: Brown 6 tbsp butter in a large pot until nutty and golden. Stir in marshmallows until melted. Remove from heat, add vanilla, and fold in rice cereal.

Press mixture firmly into the prepared pan to form an even layer.

For the caramel: In a saucepan, whisk 1/2 cup butter, brown sugar, condensed milk, and corn syrup. Bring to a boil, then simmer on low for 5-7 minutes, stirring constantly until thickened.

Pour hot caramel over the cereal base. Let cool 15 minutes, then refrigerate for 30 minutes to set.

For the top: Melt chocolate chips and oil in microwave until smooth. Pour over the chilled caramel layer.

Sprinkle immediately with flaky sea salt. Refrigerate for at least 1 hour until solid.

Let sit at room temperature for 10 minutes before slicing into squares with a hot knife.