½ cup (113 grams) unsalted butter, room temperature½ cup (128 grams) creamy peanut butter½ cup (110 grams) firmly packed dark brown sugar¼ cup (50 grams) granulated sugar1 large egg (50 grams), room temperature1 large egg yolk (19 grams), room temperature2 teaspoons (8 grams) vanilla extract1½ cups (188 grams) all-purpose flour1¼ cups (125 grams) old-fashioned oats1 teaspoon kosher salt½ teaspoon baking powder¼ teaspoon baking soda½ cup (112 grams) mini red and blue candy-coated chocolate pieces3 ounces (84 grams) white chocolate, choppedGarnish: mini red and blue candy-coated chocolate pieces, chopped white chocolate
Preheat oven to 350°F (180°C). Line 2 rimmed baking sheets with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter, peanut butter, and sugars at medium speed until creamy, 1 to 2 minutes, stopping to scrape paddle and bottom and sides of bowl. Add egg and egg yolk, beating until combined. Beat in vanilla; scrape sides of bowl.
In a medium bowl, whisk together flour, oats, salt, baking powder, and baking soda. With mixer on low speed, gradually add flour mixture to butter mixture, beating just until combined and stopping to scrape sides of bowl. Fold in chocolate pieces and white chocolate.
Using a 3-tablespoon spring-loaded scoop, scoop dough (about 60 grams each), and place 2 inches apart on prepared pans.
Bake, one pan at a time, until edges are golden brown and set, 12 to 15 minutes. (Second pan can stand at room temperature while the first bakes.) Immediately smash cookies with bottom of a glass or jar. Garnish with chocolate pieces and white chocolate, if desired. Let cool completely on pans. Store in an airtight container for up to 3 days.