Preheat oven to 325° F. Line two 8-9" round baking pans* with parchment paper and spray well with nonstick cooking spray. Set aside.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
In a measuring cup, stir together milk and vanilla extract. Set aside.
In a large bowl, beat butter, oil, and sugar together until smooth and well combined.
To the butter, sugar and oil mixture, add eggs and egg whites one at a time, mixing well after each addition. Continue mixing until the batter is light and fluffy. Scrape the sides of the bowl as needed.
Add the flour mixture and milk mixture to the batter in alternating additions, starting and ending with the flour (flour → milk → flour → milk → flour). Mix just until combined—don’t overdo it.
Divide batter evenly between prepared pans and smooth the tops. Bake for 30–40 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Let cakes cool in pans for 15–20 minutes, then turn out onto cooling racks and cool completely.
For the Filling + Frosting
In a large bowl, beat heavy cream, powdered sugar, and vanilla until stiff peaks form.
Divide the whipped cream in half. Fold diced strawberries into one half. Leave the other half plain for frosting.
Assemble the Cake
Using a serrated knife, carefully slice each cake in half horizontally to create 4 layers total.
Place one cake layer on a serving plate. Spread a layer of strawberry whipped cream over the top.Repeat with remaining layers, finishing with the final cake layer on top.
Frost the outside of the cake with the plain whipped cream. Smooth it out (rustic is totally fine here).
Top with extra strawberries. Refrigerate for at least 4 hours (or overnight) before slicing so everything sets nicely.