Preheat oven to 350F (177C) and prepare two deep 8" round cake pans** by lining the bottoms with parchment paper and lightly greasing the sides. Set 

In a large bowl using an electric mixer cream together the butter, olive oil and sugar until creamy and well-combined.

Add eggs, one at a time, beating well after each addition.

Stir in vanilla extract.

In a separate, medium-sized bowl, whisk together flour, baking powder, and salt.

Using a spatula and gently hand-mixing, alternate adding flour mixture and buttermilk to the butter mixture, starting and ending with flour mixture and mixing until just combined after each addition.  The batter should be smooth and completely combined, but avoid over-mixing.

Evenly divide batter into your prepared cake pans, and bake on 350F (175C) for 30-35 minutes.  When the cake is done, the surface should spring back to the touch and a toothpick inserted in the center should come out mostly clean with few moist crumbs (no wet batter).

Allow cakes to cool in their cake pans for 10-15 minutes before inverting onto cooling rack to cool completely