Whisk together the 1½ cups cake flour, 1½ cups all-purpose flour, 1 cup rolled oats, 1 tablespoon malted milk powder, 1 tablespoon corn starch, and ½ teaspoon baking soda in a medium bowl. Set aside.
1½ cups cake flour, 1½ cups all-purpose flour, 1¼ teaspoons salt, 1 tablespoon corn starch, ½ teaspoon baking soda
In a stand mixer fitted with a paddle attachment, cream together the 1 cup cold salted butter, 1⅓ cups light brown sugar, ½ cup granulated sugar, ½ cup peanut butter, and 1¼ teaspoons salt. Mix at medium-high speed until smooth, about 3 to 4 minutes, scraping down the sides of the bowl about every minute.
1 cup cold salted butter, 1⅓ cups light brown sugar, ½ cup granulated sugar
Once combined, add in the 2 large eggs one at a time, followed by the 3 additional large egg yolks. Then, mix in 1 tablespoon vanilla extract.
2 large eggs, 3 additional large egg yolks, 1 tablespoon vanilla extract
Once smooth, mix in the flour mixture a little at a time until smooth.
Add in the 10 ounces M&Ms, 8 ounces milk chocolate, and 1½ cups semi-sweet chocolate chips. Mix just until combined.
10 ounces M&Ms, 8 ounces milk chocolate, 1½ cups semi-sweet chocolate chips
Weigh each dough ball out to 6oz and place on a parchment-lined baking sheet and refrigerate for at least 2 hours.
Preheat the oven to 425°F (NOT A TYPO) and place two 12x17-inch baking sheets on top of each other. Line the top one with parchment paper. Double layering the baking sheets helps prevent the bottom of the cookies from overbaking.
Add 4 cookies to the lined baking sheet and sprinkle with flaky sea salt and bake for 9 to 11 minutes.
Allow them to set on the pan for a few minutes before transferring to a wire rack to cool.
Place the first top pan in the freezer to chill, and place a new pan on top of the original bottom pan and line with parchment paper and bake the next four cookies. Rotate once more to bake the remaining 4 with the sheet from the freezer.