Preheat the oven to 400°F.
Whisk together the dry ingredients in a large bowl: All-Purpose flour, cake flour, corn starch, baking soda, baking powder and salt. Set aside.
1 cup All-Purpose Flour, 1 cup Cake Flour, 1 tsp. cornstarch, 1/2 tsp. baking soda, 1/2 tsp. baking powder, 1/2 tsp. salt
Using a stand mixer fitted with the paddle attachment, cream together the butter and sugars until light and fluffy, about 3-4 minutes. 8 Tbsp. unsalted butter, cold, cut into cubes, 1/2 cup brown sugar, packed, 1/4 cup granulated sugar
Add in the creamy peanut butter, then add in the egg, egg yolk and vanilla extract. (In that order).
3/4 cup creamy peanut butter, 1 large egg, 1 large egg yolk, 1 tsp. vanilla extract (but I always add a liiiittle extra)
Gradually add in the dry ingredients followed by the M&Ms, chocolate chips, quick oats and chopped peanuts. Mix just until combined. Turn off the machine.
1/2 cup mini M&Ms, 1/2 cup chocolate chips, semi-sweet, 1/2 heaping cup quick oats, 1/2 cup chopped peanuts
Form the dough into 6 large dough balls and place on a cookie sheet. You can use a silicone baking mat or parchment paper if you prefer.
Flatten each dough ball slightly with your fingers, and sprinkle a little flaky sea salt on top of each cookie and a couple mini M&Ms, if you're feeling extra.
1 Tbsp. Flaky sea salt (for sprinkling on top of cookies)
Bake for 10-12 minutes, or until tops of cookies are golden brown. Ovens may vary. Top with more chocolate chips when they come out of the oven, if desired.
Cool on the baking sheet for 15 minutes then transfer to a cooling rack to cool completely.*