Butter and flour two 9-inch round cake pans and preheat oven to 350 degrees.

In mixer bowl, on medium-high speed, beat butter and sugar until fluffy (4 to 5 minutes).

Add eggs, then yolks,one a time, beating well after each, scraping sides of bowl occasionally.

Beat in vanilla.

In another bowl, mix flour, baking powder and salt.

Mix in about a third of flour mixture into butter mixture.

Stir in half the milk, just until blended.

Stir in another third of flour mixture, then remaining milk and finally the rest of the flour mixture.

Spread batter equally into two the two cake pans.

Bake until wooden skewer or toothpick inserted in the center comes out clean (25 to 30 minutes).

Cool on racks in pans for 10 minutes, then inverte cakes onto racks and remove pans.

Cool completely before forsting.