Giant Chocolate Chip Party Cookie

Name: Giant Chocolate Chip Party Cookie
Ingredients: Nonstick cooking spray; 2 1/4 cups all-purpose flour; 1/2 cup oatmeal, ground in a food processor; 3/4 teaspoon baking soda; 1/2 teaspoon fine salt; 1/8 teaspoon ground cinnamon; 1 stick (4 ounces) unsalted butter, softened; 1 cup light brown sugar; 1/2 cup granulated sugar; 2 teaspoons pure vanilla extract; 2 large eggs; 1 cup semisweet chocolate chips; 12 to 15 mini peanut butter cups; 6 tablespoons unsalted butter, at room temperature; 1 cup confectioners' sugar; 2 teaspoons milk; Suggested decorations: melted chocolate for drizzling and confetti sprinkles