Preheat your oven to 350 degrees, and grease three 6-inch round cake pans and line the bottoms with parchment paper. This recipe also works with two 8-inch round pans.
In a large bowl, beat together the butter, oil, and sugar until fluffy. Add in the vanilla extract, sour cream, and egg yolks, and beat until combined. Add in the flour, baking powder and salt, and beat until combined, then beat in the milk. The batter will be fairly thick.
In a separate bowl, beat the egg whites until they just start to become fluffy, and then add in the remaining sugar VERY slowly as you continue to whip the egg whites. Beat until you reach stiff peaks.
Stir about 1/3 of the egg whites into the batter to lightly the base of the batter. Then fold in the remaining egg whites. This will take a bit of time, but keep folding until the whites are fully incorporated.
Divide the batter evenly between your pans, and bake for 18-20 minutes, or until a cake tester comes out clean. Cool the cakes completely before frosting.
In a food processor or blender, blend the hulled strawberries until they are mostly mashed. Pour into a small saucepan, and cook over medium heat with the sugar, cornstarch, and salt, until the mixture is thick. Cool completely.
In a large bowl or stand mixer, using a whisk attachment, beat the heavy cream, sugar, and vanilla together until stiff peaks form.
In a separate small bowl, beat the cream cheese for several minutes until smooth and fluffy. Add the cream cheese to the beaten heavy cream mixture, and whip for several minutes to incorporate fully.
Once the cake layers have cooled completely, you can trim off the tops to make even layers. Using a piping bag filled with the Chantilly cream, make a thick ring around the edge of the first layer as a dam. Add about half of the strawberry sauce to the middle, and spread evenly. Cover the strawberry sauce with more cream, and spread evenly. Top with raspberries and blackberries.
Repeat this process until all three layers are build. Then, use remaining Chantilly cream to frost the outside of the cake. Top with any remaining berries, and remaining strawberries. Keep this cake chilled until ready to serve. Enjoy!