Place butter, cream cheese, and sugar in mixing bowl and mix on low. Scrape down sides of bowl and add egg yolks and vanilla. Add flour and salt, mix until everything is well incorporated.
Take a 9x12 baking pan, and cover in parchment paper. Scoop cookie dough into center and taking wax paper (or more parchment paper covered in butter), place on top and use to press dough into pan.
Bake at 325 for 20 minutes, or until sides are slightly brown. Cooking time may vary depending on altitude and climate. Watch carefully. It's better to undercook slightly.
Let bars cool completly before attempting to lift them out of the cookie sheet. Chill slightly if possible. Tip pan and carfully slide bars out with partchment paper.
Trim off sides before frositing. Frost, cut into 2" x 2" pieces. Enjoy!