Preheat the oven to 325 degrees, prep a 9x13 pan with a swipe of shortening and dust of flour, set aside.
In a large bowl, hand whisk together the egg whites, egg, oil, vanilla, almond extract, sour cream, and buttermilk until thoroughly combined. Sift in the cake mix and gently stir until it's combined (don't overmix). Toss in the berries and gently fold in.
Spread the batter evenly into the prepared baking sheet. Bake for 27-30 minutes. Let cool completely before adding on the other cake elements. Make the oatmeal crumble, filling, and buttercream ahead of time or while the cake is baking.
For the Berry Filling
Stir everything except for the cornstarch in a saucepan over medium heat until the mixture begins to bubble. Stir and let cook for about 5 minutes until the berries reduce slightly, then add in the cornstarch. Stir until thickened, then store the filling in a bowl to cool completely in the fridge/freezer before slathering onto the cake.
For the Oatmeal Crumble
Preheat the oven to 350 degrees. In the bowl of a stand mixer fitted with a paddle attachment, combine all the ingredients until a light crumble forms. Spread the crumble out onto a sheet pan and bake for 10 minutes, then stir. Bake for another 5 minutes until lightly toasted, then let cool completely before sprinkling onto the cake.
For the Buttercream
In a bowl of a stand mixer fitted with a paddle attachment, whip up the butter until it's light and fluffy. Add in the salt, vanilla, cinnamon, and heavy cream, and whip it up until everything is combined. With the mixer on low speed, slowly add in the powdered sugar. Flip the speed to high and whip up the buttercream until it's light and fluffy in texture and color. Find a 1M and small circle piping tip. Fill one bag with the 1M (about 2 cups) and buttercream, and another piping bag with the circle piping tip and buttercream. Set aside.
Assembly
Slather on a thin layer of buttercream on the sheet cake with an angled icing spatula.
Slather on all the filling, then sprinkle on the oat crumble. Press down gently to make it as flat as possible.
Pipe on the lines as shown in the video above with the circle piping tip, then pipe a shell border around the outside with the 1M piping tip. Enjoy!