Preheat oven to 325 degrees F. Line non stick baking pan, 9x13, with parchment.
Put dry ingredients(flour, baking powder and salt) into bowl and stir with whisk to combine.
Cream butter until light and fluffy, add in sugar. Mix until light and pale yellow.
With mixer on medium speed, stream in the oil.
Add one egg at a time, mixing well after each addition.
Add vanilla to buttermilk.
Start adding in your flour mixture and alternate with your buttermilk. Make sure you end with the flour mixture. Scrape down bowl as needed.
Pour into prepared pan and bake for 32-35 minutes or until toothpick inserted into center comes out clean. Cool completely.
In bowl of mixer cream butter until light and fluffy.
With mixer on low, slowly add in powdered sugar one spoonful at a time. Alternating with the milk as needed. Scrape down bowl as needed.
Add in the vanilla. Mix until completely combined and buttercream is light and fluffy.