Line an 8x8 *pan with foil. Gently cover with cooking spray. Set aside.
In a large pot, melt the butter over medium low heat. Once melted add 8 cups of the mini marshmallows to the pan. When the marshmallows are just melted, add the vanilla, kosher salt, and Rice Krispies cereal. Remove from heat. Stir.
Once the Rice Krispies cereal is mostly covered with the melty marshmallow mixture, add the remaining 2 cups of mini marshmallows and the M&M's. Stir to combine.
Place the Rice Krispies treat mixture into the foiled pan. Gently press into an even layer. **
Let set. Enjoy!
*an 8x8 pan will yield thicker Rice Krispies. Using a 9x9 inch pan works perfectly too. Use whatever you have on hand. ** When pressing the Rice Krispies down, don't press too hard! You don't want them to be too compact. Also, if the mixture is sticking to your hands, gently rub butter onto your hand and then use your hand to press the mixture into the pan. It might be hot so be careful! But this will prevent it from sticking to you :)Recipe Source: adapted from Cookies and Cups.