Preheat your oven to 350 degrees F. Butter and flour an 18-by-13-inch rimmed baking sheet. In a mixing bowl, whisk together the flour, cornstarch, salt, baking soda, and baking powder, then set aside.

In the bowl of an electric stand mixer fitted with the paddle attachment, whip the softened butter, granulated sugar, and lemon zest together until the mixture is pale and fluffy. Mix in the eggs one at a time, then mix in the vanilla extract.

Add one-third of the flour mixture and mix just until combined. Add half of the sour cream and mix until combined. Repeat this process once more, ending with the final third of the flour mixture. Scrape down the sides of the bowl to ensure everything is evenly incorporated.

Pour the batter into the prepared baking sheet and spread it evenly. Bake in the preheated oven for 20 to 24 minutes, or until a toothpick inserted into the center comes out clean.

Remove the cake from the oven and let it cool completely on a wire rack. Once cool, cut the cake into 24 rounds that match the diameter of your serving cups (you will have some leftover cake pieces, which can be saved or frozen for snacking).

Toss the diced strawberries and blueberries with the sugar in a mixing bowl. Let the berries sit at room temperature for about 10 minutes to release their sweet, natural juices.

In a large mixing bowl, use an electric hand mixer to whip the heavy cream until soft peaks form, then set aside.

In a separate medium bowl, whip the softened cream cheese until it is completely smooth. Add the powdered sugar and vanilla extract, and whip until the mixture is light and fluffy.

Gently fold the cream cheese mixture into the whipped heavy cream, then whip briefly until stiff peaks form.

Place 12 cake rounds into the bottoms of 12 dessert glasses or cups. Spoon a generous layer of the macerated berries and their juices over each cake round.

Transfer the whipped cream mixture to a piping bag fitted with a large round tip. Pipe a layer of cream over the berries in each cup.

Repeat the layers once more: place another cake round on top, follow with more berries, and finish with a final layer of piped cream.

Garnish each trifle with a whole strawberry and a few fresh blueberries. Serve cold.