Butter a 15x10-inch sheet pan.
In a large pot, melt the butter and peanut butter over low heat. Add the marshmallows and salt. Cook, stirring, until mixture is melted, about 5 minutes. Remove from heat and immediately stir in the cereal until coated.
Quickly transfer to the prepared pan. Using greased hands, press the mixture into a flat, even layer. Let stand for 15 minutes.
Cut the treats in half width-wise. Spread the strawberry jam in an even layer over one half of the treats. Carefully transfer the second layer to fit evenly over the jam. Cut into 12 “sandwiches.”