Melt 1/2 cup of the butter in a medium pot over medium heat (reserving remainin 1/4 cup for later), then add peanut butter and butterscotch chips. Heat until peanut butter and butterscotch chips are melted.
Add marshmallows and continue to heat and stir until marshmallows are melted and mixture is well blended.
Place oat cereal in a large bowl and then pour liquid mixture over top. Mix well with a wooden spoon, then press into a 8x11 pan lined with parchment paper. Press with the back of your spoon or your slightly dampened hand. Set aside.
Place chocolate chips and reserved 1/4 cup butter in a microwave safe bowl and microwave in 20 second intervals, removing bowl between intervals to stir. Chocolate is ready when it is smooth. You can also melt over a double boiler on the stove, or a bowl placed over a pot of simmering water.
Working quickly, pour melted chocolate over the oat squares and smooth out with rubber spatula. Place in fridge for about 10 minutes until chocolate is beginning to set, then remove and sprinkle with the kosher salt. Return to fridge and allow chocolate to fully set, then cut into squares