Line a 9x13-inch pan with parchment and lightly butter it. Set aside.
Melt butter in a large pot over medium-low heat until just fluid.
Add marshmallows and cook, stirring, until smooth. Remove from heat; stir in vanilla and kosher salt.
Fold in cereal and about half the chopped pistachios until evenly coated.
Press mixture into the pan with a greased spatula. Don’t pack too firmly.
Melt dark chocolate with oil in a microwave in 20–30 second bursts, stirring until glossy.
Spread or drizzle chocolate over the bars. Sprinkle remaining pistachios and flaky salt on top.
Let set at room temperature until firm, then cut into 16 squares.