Lightly spray a 9×13 inch dish with baking spray. Line with parchment paper leaving excess on two sides (this will make the treats easier to remove from the pan once finished). Lightly spray the parchment paper with baking spray.
Melt the butter in a large pot over medium low heat. Stir occasionally at first. Once the butter has melted, stir occasionally with a whisk. It will begin to bubble. Once it begins to foam, whisk it constantly as you watch for it to turn a light, amber color then light golden brown. Milk solids will begin to sink to the bottom of the pot. They'll start to turn light brown and then the butter will begin darkening and will develop a nutty smell. When this happens, remove the pan from the heat. Keep whisking since the residual heat will turn those brown bits into an amber color. There will be some little light brown bits on the bottom of the pan. That will add flavor. Leave the brown butter in the pot and continue with the next step immediately.
Stir 9 ¼ cups of mini marshmallows into the brown butter until the marshmallows have melted.
Next, stir in the vanilla extract and salt.
Stir in the Rice Krispies until the Rice Krispies are coated with the marshmallow butter mixture. Allow to cool in the pot for 5 minutes. Once the mixture has cooled slightly, fold in the remaining 1 ¾ cups of marshmallows followed by the M&Ms.
Turn the mixture out onto the pan. Use your (greased or slightly wet) hands, a greased spatula or a leftover butter wrapper to gently spread and press the mixture evenly into the pan. Avoid pressing down too much as this can make the treats hard.
Let the rice krispie treats set for about 30 minutes.
Use parchment paper to lift them out of the pan. Drizzle or spread the chocolate over the whole thing or first slice them into squares then cover with chocolate individually. Enjoy!