Spray a 9x13 glass pan with non-stick spray. Also spray a rubber scraper.
In a large bowl measure out marshmallows and butter. Microwave for about 1 minute 15 seconds, until marshmallows are puffing up.
Stir the Rice Krispie cereal into the marshmallow mixture. Once well mixed, transfer to the 9x13 pan and gently press in until the treats are as even as possible.
Once they're cooling in the pan, pour the caramel bits into a microwave-safe dish and add the milk. Microwave for about 1 minute 20 seconds. Bits won't necessarily look melted, but once you begin stirring them, they'll run smooth. If they don't, put back in the microwave for another 10 seconds.
Pour caramel over the Rice Krispie Treats and use your rubber scraper to smooth the caramel over the top until even.
Sprinkle on coarse sea salt and let cool completely.
Once cool, I like to take all the treats out and put the entire block on a cutting board. I slice them all at once, then return them back to the pan.