In a medium bowl whisk together 2 1/2 cups flour, baking powder, and kosher salt. In a large bowl whisk the sugar and eggs until thick and pale, about 1 minute. Slowly add the vegetable oil and melted butter, and mix until combined. Add the buttermilk and vanilla and mix until well combined.
2 1/2 cups All-purpose flour, 2 1/2 tsp Baking powder, 1 tsp Kosher salt, 1 cup Granulated sugar, 2 large Eggs, 4 tbsp Unsalted butter, 1/4 cup Vegetable oil, 1 cup Buttermilk, 2 tsp Vanilla extract