Preheat oven to 350 degrees F (177 degrees C) and place the rack in the center of the oven. Butter, or spray with a nonstick baking spray, an 8 x 8 inch (20 x 20 cm) square baking pan.
In a small saucepan, melt the butter. Remove from heat and let cool to room temperature.
In a small bowl, whisk together the flour, salt, and baking soda.
In the bowl of your electric mixer (or with a hand mixer), beat the melted butter, white sugar and brown sugar until light and pale (2-3 minutes).
Beat in the egg and vanilla extract. Scrape down the sides of the bowl then, on low speed, beat in the flour mixture until incorporated.
Fold in the m&ms ,sprinkles and white chocolate chips until just combined. Make sure not to over mix otherwise the sprinkles would discolor the batter.
Pour the batter into the prepared pan, smooth the top, and bake for about 23 - 25 minutes, or until the top of the bars are dry, golden brown, and a toothpick inserted in the center comes out with a few moist crumbs.
Remove from oven and place on a wire rack. Cool completely before cutting into squares.