Preheat oven to 375°F [190°C]. Line two baking sheets with parchment paper.
Cream the butter, peanut butter and sugars together in the bowl of a stand mixer fixed with the paddle attachment until fluffy and smooth, about 4 to 6 minutes. Scrape down the sides of the bowl as needed. Add the egg and vanilla and continue beating until smooth.
Reduce mixer speed to low and gradually incorporate the flour, baking soda and salt. Once fully incorporated, fold in 1 cup of the M&Ms and the Rice Krispies. Turn the mixer off immediately once all ingredients are mixed in (over mixing your dough will make the cookies tough).
Use a 1 Tbsp scoop to drop the dough 2 in [5 cm] apart on the prepared baking sheet. Leaving space between the cookies is important, they will spread as they bake. Press 2 to 3 M&Ms into the top of each cookie.
Bake for 8 to 10 minutes, or until the edges have lightly browned but the tops are still very blonde. They should look undercooked! This is how to ensure they stay nice and soft. Allow the cookies to cool slightly, then transfer to a wire rack to cool.
