Spring Confetti Cookie Bars

Name: Spring Confetti Cookie Bars
Ingredients: 1 1/2 cups all-purpose flour; 1/2 teaspoon baking soda; 1 teaspoon baking powder; 1/2 teaspoon cornstarch; 1/4 teaspoon salt; 1/2 cup room temperature butter; 1/2 cup white sugar; 1/4 cup firmly packed brown sugar; 1 large egg (room temperature); 1 1/2 teaspoons pure vanilla extract; 1 1/4 cup white chocolate chips (separated); 3 cups miniature marshmallows; 3/4 cup pastel M&Ms (or other candy); 1/2 cup broken pretzel twists; 3 tablespoons sprinkles (pastel colored)