Preheat the oven to 375 degrees F, and line baking sheets with parchment paper.
In a large bowl with an electric mixer, cream the butter, peanut butter, brown sugar, and granulated sugar together until pale and fluffy (3 to 5 minutes on medium-high speed).
Stir in the egg, egg yolk, and vanilla until well-blended (about 1 minute on medium speed).
Add the flour, oats, cornstarch, baking soda, and salt, and mix in until incorporated (about 30 seconds to a minute on low speed).
Stir in the chocolate chips and m&m's, and scoop 1.5-tablespoon balls onto the prepared baking sheets.
Bake for 6 to 9 minutes, or until set around the edges, slightly golden on the bottoms, and still soft in the centers.
Recipe Video
Recipe Notes
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