Melt and brown 10 tablespoons unsalted butter in a light skillet over medium heat until amber bits form, 4–6 minutes; remove from heat and cool to 110–120°F.

Whisk together 2 1/4 cups all-purpose flour, 1/2 teaspoon cornstarch, 1 teaspoon baking soda, 1/2 teaspoon baking powder, and 1/2 teaspoon kosher salt, then stir in 1 1/2 cups rolled oats and set aside.

In a mixer, combine 1 cup packed light brown sugar and 1/2 cup granulated sugar; add 2 large eggs and 2 teaspoons vanilla and mix until blended.

With the mixer on low, slowly add the cooled browned butter until just combined; avoid overmixing.

Fold the dry ingredients into the wet just until combined, then stir in 1 cup red and blue candy-coated chocolates, 1 cup semisweet chocolate chips, and 1/2 cup chopped nuts if using.

Scoop the dough with a 4-tablespoon scoop onto a parchment-lined baking sheet, spacing 3 inches apart; chill the scooped dough for 30–60 minutes for best spread control.

Preheat the oven to 350°F with a rack in the center. Bake chilled scoops 12–15 minutes until edges are golden and centers look slightly glossy but set.

Cool the cookies on the sheet for 8–10 minutes before transferring to a cooling rack to finish setting.