Preheat the oven to 350°F / 175°C. Butter a 7" springform pan*, then coat the inside with sugar.
To a medium sized mixing bowl, add the flour, baking powder, salt, sugar, and cubed butter. Use a hand mixer or a stand mixer fitted with the paddle attachment to beat the mixture together until the butter has completely combined with the dry ingredients and no lumps remain.
Add in the oil, vanilla, sour cream and milk. Whisk the ingredients together just until a cohesive batter has formed.
In a separate bowl, beat the egg whites until they reach stiff peaks.
Add the egg whites to the batter and gently fold them into the batter until the mixture is cohesive.
Pour the cake batter into your prepared pan. Bake for 35-45 minutes, until a toothpick inserted in the center of the cake comes out with only a few moist crumbs.
Let the cake cool in the pan for 10-15 minutes, then put it out onto a wire rack to finish cooling.