Grease an 8 x 8 glass baking dish with butter and set aside.
In a large pot on very low heat add the butter. Once melted add marshmallows and cocoa power, stir until melted and smooth.
Take off heat, stir in vanilla and salt, mix to combine.
Gently stir in Rice Krispie cereal and 1 cup dark chocolate chunks until combined, pour into the prepared glass baking dish and gently spread the Rice Krispie mixture out evenly in the pan.
Let sit on the counter until cooled and firm, about 30 min. Cut into squares, sprinkle with remaining 1/4 cup dark chocolate, sprinkle with sea salt and enjoy.