Chocolate Covered Peanut Butter Rice Krispie Treat Brownies

unsalted butter, cut into pieces, plus more for greasing pan

+ 1/2 cups semi-sweet chocolate chips, divided

granulated sugar

packed light-brown sugar

salt

eggs, at room temperature

all-purpose flour

+ 1/2 cups Rice Krispies cereal

butter

(about 5 ounces) regular marshmallows or 2 heaping cups miniature marshmallows

creamy peanut butter

Rice Krispies cereal

+ 1/2 cups chopped milk or semi-sweet chocolate (I like Callebaut )

melted shortening