Prep a 8" square baking pan. Spray/grease or line with foil (greased) or parchment paper.
Stir together cookie mix, softened butter, egg & egg yolk until soft dough forms.
Spoon & evenly press dough into baking dish.
Bake until corners are just golden brown - 17 - 25 minutes. Lower times will be for a metal tin, longer times for a glass dish.
Remove from oven & cool on wire rack.
*Note* I line baking tin with aluminum foil (lightly sprayed with cooking spray) and let extra hang over the edges. Once cooled, you can lift the foil and bars straight out of the tin. Works wonderful with parchment paper as well. It's not necessary - but it makes bars/brownies neater & easier to frost & cut.
For buttercream:
In a large mixing bowl, cream butter with electric mixer.
Beat in vanilla.
Mix in sugar, one cup at a time. Beat well on medium speed, scraping sides and bottom of bowl.
Icing will be dry when all sugar has been mixed in.
Add in milk, beat at medium speed until light and fluffy.
Mix in any colors if desired.
Frost top of cooled sugar cookie bars. Cut into 16 pieces.