Step 1

Preheat oven to 350ºF and line 2 large baking sheets with parchment paper or silicone baking mats. In a large bowl, cream together the butter, brown sugar, and peanut butter with a hand or stand mixer until light and fluffy, about 3 minutes. Mix in the egg and vanilla, scraping down the sides of the bowl as needed.

Step 2

In a separate bowl, add the flour, baking soda, cornstarch, baking powder, and salt; whisk to combine. With the mixer on low speed, slowly add the flour mixture to the wet ingredients and mix until just combined. Stir in the oats, chocolate chips, and M&M candies by hand. The dough will be a bit soft and sticky.

Step 3

To make it more manageable to work with, allow the cookie dough to sit for 15 minutes – the oats will absorb some moisture, making the dough easier to work with. It's not necessary, but you can also chill the dough for about 30 minutes to make it less sticky. Drop the cookies by heaping spoonfuls onto the prepared cookie sheets, about 2 tablespoons of dough per cookie, leaving 2 inches in between cookies. For presentation, you can dot a few extra M&Ms and chocolate chips on each mound of dough before baking.

Step 4

Bake for 9-10 minutes, or until the cookies are just starting to brown around the edges and the top still looks slightly undercooked. Do not over bake - cookies will continue to bake on the cookie sheet. Remove from the oven and cool for 5-10 minutes on the baking sheet before moving them to a wire rack to cool completely. Cookies will firm up as they cool.