01 - Preheat your oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper, ensuring an overhang for effortless removal.
02 - In a large mixing bowl, thoroughly whisk together the flour, granulated sugar, baking powder, baking soda, and fine salt.
03 - In a separate bowl, whisk together the melted butter, eggs, vanilla extract, whole milk, and sour cream until a smooth emulsion forms.
04 - Gently fold the wet ingredients into the dry ingredients until just combined. Exercise caution to prevent overmixing.
05 - Carefully fold in the white chocolate chips, 1 cup of rainbow sprinkles, and the dry confetti cake mix until they are evenly distributed throughout the batter.
06 - Pour the prepared batter into the lined baking pan and smooth the surface evenly.
07 - Generously scatter the mini marshmallows and the remaining rainbow sprinkles evenly over the top of the batter.
08 - Bake for 25 to 28 minutes, or until the edges exhibit a golden hue and a toothpick inserted into the center emerges clean or with a few moist crumbs.
09 - Allow the bars to cool completely in the pan before lifting them out using the parchment paper overhang. Cut into desired bar portions.