Preheat oven to 350 degrees. Line a 9x13 pan with parchment paper and set aside.
In a large mixing bowl, combine butter, sugar, and vanilla. Beat in eggs until light and fluffy, about a minute.
In a separate bowl, sift together dry ingredients - flour, baking soda, and salt. Slowly stir dry ingredients into wet ingredients until combined.
Add rainbow sprinkles and stir to combine.
Pour into prepared pan and use a spatula to spread evenly to the edges. Batter will be thick.
Sprinkle up to ¼ cup of reserved granulated sugar evenly over the top of the batter.
Bake for 20-22 minutes, then test with a toothpick for doneness. Insert toothpick into the center of the pan. If toothpick comes out with wet batter, the bars are not done. Continue checking every 2 minutes. If toothpick comes out with a few crumbs, then the bars are done. Bars will be lightly browned around the edges and beginning to lightly brown on top.
Place pan onto a wire rack and allow to cool completely before cutting and enjoying.
Recipe can he halved and prepared in an 8x8 pan instead. Bake for 3-4 minutes less than for 9x13 pan.