Preheat oven to 350 degrees. Line cookie sheets with parchment paper or use a silpat liner.
In a large bowl (or bowl of a stand mixer) cream together the butter, brown sugar, and peanut butter until light and fluffy, about 2-3 minutes.
Mix in the egg and vanilla, scraping down the sides of the bowl as needed.
In a separate bowl, add the flour, baking soda, cornstarch, baking powder, and salt and whisk to combine.
With the mixer on low speed, slowly add the flour mixture to the wet ingredients and mix until just combined.
Add the oats, chocolate chips, and M&M candies. Mix on low speed just until dough is combined.
Use a small/medium cookie scoop, or about 1-2 tablespoons cookie dough, and place on the baking sheets. They should look like golf balls.
Bake the cookies for 7-9 minutes until the edges are just starting to brown and the top still looks slightly undercooked.
Remove from the oven and cool for 5 minutes on the baking sheet before moving them to a wire rack to cool completely.
** For more of a cookie shape, take the bottom of a glass cup and gently press down on the cookie a few minutes after they come out of the oven OR just press some additional m&m's on tops of the cooked cookies and that will naturally press them down into more of a cookie shape, I always do this and it works so well. The cookies don't spread much at all while baking.