Preheat oven to 325*
Butter and flour a 10" bundt pan.
Using a Kitchenaid or hand mixer combine butter, sugar, vanilla, and 4 whole eggs until well creamed.
Slowly add in flour and baking powder.
Whisk the egg yolks, zest, and buttermilk together, then slowly add mixture to rest of batter.
Mix until well blended.
Pour batter into prepared bundt pan.
Bake at 325* for 1 hour or until thin knife inserted into center of cake comes out clean.
Cool in pan on wire rack.
Remove from pan and dust with powdered sugar.