• Preheat oven to 375° F.

  Lightly butter, or line with parchment paper, an 8-inch square baking pan leaving some overhang to let you easily remove the bars later.
• In a large bowl, using a stand mixer fitted with a paddle attachment, or hand-held mixer, cream the butter for about 1 minute. Add the sugar and beat until light and fluffy, about 2 more minutes. Scrape the side of the bowl occasionally.
• Beat in the whole egg, egg yolk, and vanilla until just combined.
• Turn off the mixer and add the flour and salt. Blend gently just until combined and no streaks remain. Scrape bowl if needed.
• Add in most of the sprinkles (about 2/3rds), if using, and mix with a rubber spatula. (The sprinkles could shatter if using the mixing blade to mix them in)
• Press the batter into the prepared pan and evenly smooth the top and then scatter the remaining sprinkles on top.
• Bake for 21-23 minutes or until lightly browned on the edges and top. The top should no longer be shiny.

  These bars are supposed to be soft so be careful not to overbake them because every oven bakes differently.
• Remove from the oven and allow the pan to cool completely on a cooling rack. The middle may sink a little but that is normal.
• Remove the entire square from the pan by loosening the edges and lifting them out or use the parchment overhang. Place on a cutting board.

  Cut into about 16 pieces.
• If frosting the bars it's easier to frost them before cutting.

Storage:

• Store in an airtight container and keep at room temperature for up to 5 days.