Prep: Make sure all cold ingredients have come to room temperature. Preheat your oven to 350° F. Prepare two 9-inch cake pans. Use a paper towel to rub shortening all over the inside of each pan, coating them evenly & completely. Cut a round of parchment paper to fit inside each (or purchase pre-cut rounds). Place this on top of the greased pan bottom, then grease over the parchment paper as well. Add a handful of flour, and tap the pan around to evenly coat the entire inside with flour, tapping out the excess.
Make the Cake Batter: In a medium bowl, whisk together the dry ingredients – flour, baking powder, baking soda, and salt. In a large mixing bowl, cream the softened butter for about 30 seconds, until creamy. Add the vegetable oil and beat together for 1 minute. Add the sugar and beat on high speed for 2 full minutes. The mixture will be fluffy. Scrape the sides of the bowl down throughout, to ensure everything is combined. Add the egg whites, two at a time and then the last one on its own, beating on medium speed between each addition, for 45 seconds. The mixture will be very fluffy. Add the sour cream and vanilla and mix on medium low speed until just combined.Add half of the dry ingredients with the mixer running on low speed. Add the milk with a few streaks of flour remaining, then when the milk is almost mixed in, add the remaining dry ingredients. When just a few streaks of flour remain, turn off the mixer and use a spatula to make sure everything is combined – don't over mix! Bake: Divide cake batter evenly between the pans. (You can eyeball this or use a kitchen scale - 656 grams per prepared pan.) Bake the cakes for 21-26 minutes, rotating the pans halfway through (my oven has hot spots, so I need to do this). A toothpick inserted into the center will pull out moist crumbs. Let cakes cool in the pans on cooling racks for 10 minutes. Then, invert onto cooling racks to cool completely.
Prep the Berries: Add berries, sugar, and jam to a medium bowl and let rest while you prepare the next item. This will allow the juices to start flowing.
Make the Whipped Cream: Add all ingredients to a large bowl and beat on medium high until stiff peaks form, then stop mixing.
Assemble: Place one cake layer on a cake plate or cake stand, and spread with half the whipped cream. Add slightly less than 1/2 the berries, spooning on some of the juices. Repeat with the second layer and remaining components, piling the berries high on top.
Serve & Store: Slice and serve immediately, or chill in the fridge for 30 to 60 minutes. Store leftovers in the fridge in an airtight container, with plastic wrap pressed over the cut portion of the cake, for 3-4 days.
Make-Ahead Tips: I do not recommend assembling this cake a day ahead, or making the whipped cream ahead, as it is best served fresh. Cake layers can be baked, cooled, and stored on plates tightly wrapped in plastic wrap a day ahead of assembling. Mix the berries up the day of, as well.