Preheat the oven to 350 Fahrenheit.
Line an 8x8 pan with parchment paper and set aside.
In a mixing bowl, combine the butter and sugar. Beat with an electric mixer for about 30 seconds.
Add in the egg, egg yolk and vanilla. Beat until combined.
Add in the dry ingredients and beat until combined.
Transfer to your prepared baking pan and spread the top out smooth.
Bake for 23-27 minutes or until a toothpick comes out clean and the edges are just golden brown.
Let cool. Meanwhile, combine the butter for the frosting and the sugar with an electric mixer until fluffy. Beat in the vanilla and milk, one tablespoon at a time until your desired consistency is reached.
Add food coloring if desired or leave as is. Top the cooled bars with the frosting and sprinkles. Slice into 16 squares and enjoy!