Melt butter and sugar in a microwave-safe bowl for 60 seconds. Stir.
Add 2 1/2 cups mini marshmallows and microwave in 30-second intervals for 60-90 seconds, stirring after each interval until melted and smooth.
Stir in cereal and continue to stir until coated evenly.
Stir in the remaining (1 cup) of mini marshmallows
Press mixture into a buttered 9"x13" baking dish (or baking dish lined with parchment paper). Sprinkle with sea salt, to taste.Allow Rice Krispies treats to cool.
Slice into squares in the pan, or pull up the parchment paper sides to remove Rice Krispies from the pan and slice into squares on a cutting board.
Video
Notes
Stove top directions: heat butter, brown sugar, and marshmallows on medium to low heat until melted and smooth. Stir in cereal and mini marshmallows using a wooden spoon. Prepare the rest the same as above.