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.