Simmer strawberry puree, sugar, and lemon juice for 10–15 minutes. Stir in cornstarch slurry and boil 1 minute until thick. Cool completely.
Preheat oven to 350°F. Mix dry ingredients, beat in butter until sandy. Add milk, sour cream, egg whites, and vanilla. Divide into pans and bake 25–30 minutes. Cool fully.
Whip heavy cream, powdered sugar, vanilla, and pudding mix until stiff peaks form.
Place first layer on stand. Pipe a whipped cream dam, fill center with strawberry reduction and fresh berries.
Repeat layers, frost exterior with remaining whipped cream.
Decorate with strawberries. Chill at least 30 minutes before slicing.
Notes
This cake is sturdy enough for stacking and holds beautifully thanks to the stabilized whipped cream frosting.