Preheat the oven to 325°F (163C) convection (300F/149C conventional) and grease and line a 9x13x2" cake pan with baking spray and parchment paper.
In a medium bowl, whisk together the flour, baking powder and salt until they're evenly distributed.
2 1/4 cup (270g) cake flour, 1 Tbsp (10g) baking powder, 1/2 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 2 minutes, scraping the edge of the bowl halfway through.
1/2 cup (113g/1 stick/4oz) unsalted butter, 1 1/2 cups (300g) granulated sugar
Add in the oil and beat for another 1-2 minutes, until it's pale and fluffy.
1/4 cup (59mL) vegetable oil
Add the egg whites, 1-2 at a time, beating on medium speed between each addition, until each egg is fully incorporated and the batter is fluffy.
5 (160g) large egg whites
Add the vanilla extract and sour cream and beat just until they're combined.
2 tsp vanilla extract, 3/4 cup (170g) sour cream
Add half of the dry ingredients to the batter, turn the mixer up to low speed and then pour in the cream, followed by the rest of the dry ingredients.
3/4 cup (177mL/6oz) heavy whipping cream
Spread the cake batter in the prepared cake pan and bake for 40 minutes.
When the cake is fully baked it will release itself from the edge of the pan and it will spring back if you press on the center of it.
Allow the cake to cool completely before frosting.