Preheat the oven to 350F/177C convection (325F/163C conventional) and grease and line three 10 inch cake pans.
In a medium bowl, whisk together the flour, baking powder and salt until they're evenly distributed.
4 1/2 cups (540g) all-purpose flour, 1 Tbsp + 2 tsp (17g) baking powder, 1 tsp salt
Place the room temperature butter and the granulated sugar in a large bowl and use an electric mixer with the whisk attachment to beat them for 1-2 minute. Add the oil and beat at full speed for another minute.
1/2 cup (118g) unsalted butter, 3 cups (600g) granulated sugar, 1 cup (237mL) vegetable oil
Then add the eggs, one at a time, beating on medium speed, for a minute, until each egg is fully incorporated. Add the sour cream and vanilla extract and beat just until combined.
6 large eggs, 1 1/2 cups (340g) sour cream, 2 tbsp vanilla extract
Add half of the dry ingredients to the wet ingredients, turn the mixer on low speed and then pour in the milk, followed by the rest of the dry ingredients.
1 1/2 cup (355mL) milk
Distribute the batter evenly among the three cake pans, about 28oz in each, and bake for 35 minutes. When the cake is fully baked it will release itself from the edge of the pan and it will spring back when you press on the center of it.
Allow the cake to cool completely before frosting.