Preheat the oven to 350F (180C) degrees. If you have a convection oven, the temperature should be 325F (160C) - some convection ovens do this automatically.
Line the bottom of two 8-inch round pans (with high sides - at least 1.5 inches) with parchment paper. Lightly grease the sides and sprinkle the sides with flour. Or brush the sides with cake pan release. *See recipe notes for additional pan sizes.
In a medium bowl sift together the flour, baking powder, baking soda and salt. Whisk the ingredients together (about 5-10 seconds). Set aside.
In a large bowl, beat the butter and sugar until fluffy - about 2-3 minutes. Turn off the mixer and scrape down the sides and bottom of the bowl.
Mix in the oil and vanilla extract.
Mix in the eggs 1 at a time, continuing to turn off the mixer and scraping down the sides of the bowl.
With the mixer on low speed, mix in about 1/3 of the flour mixture. Then mix in the sour cream.
Turn off the mixer and scrape down the sides of the bowl. With the mixer on low speed, mix in 1/3 of the flour mixture, followed by the milk. Turn of the mixer and scrape down the sides of the bowl. Then mix in the rest of the flour. Stop beating as soon as the everything is combined.
Spoon the batter evenly into the prepared pans and smooth the top. You can weigh the pans to ensure that they're the same side. Gently drop the cake pans on the counter (from about 1 inch above) to remove any excess air bubbles.
Bake in the preheated oven with both pans on the middle rack. The cakes will bake for about 30 minutes, or until an inserted toothpick comes out clean.
Cool the cakes in the pan for 10-15 minutes, then carefully insert the cake pans onto a cooling rack to cool the cakes out of the pan.