Prep the pan and oven: Heat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Macerate the strawberries: In a bowl, toss diced strawberries with sugar and lemon juice. Let sit 10–15 minutes to draw out juices. Drain off excess liquid and set aside.

Mix the wet ingredients: In a large bowl, whisk melted butter, granulated sugar, and brown sugar until glossy. Whisk in the egg, egg yolk, and vanilla until smooth.

Combine dry ingredients: In a separate bowl, whisk flour, baking powder, and salt. Add to the wet ingredients along with sour cream and lemon zest. Fold just until no dry streaks remain. The batter will be thick.

Bake the base: Spread batter evenly in the prepared pan. Bake 20–24 minutes, until the edges are set and a toothpick comes out mostly clean with a few moist crumbs. Do not overbake.

Cool completely: Set the pan on a rack and let the base cool to room temperature. A slightly warm base can melt the topping, so be patient.

Make the whipped topping: In a mixing bowl, beat cream cheese until smooth. Add cold heavy cream, powdered sugar, and vanilla. Whip to medium-firm peaks. It should hold shape without being stiff.

Assemble: Gently spread the macerated, drained strawberries over the cooled base in an even layer. Dollop the whipped topping over the berries and spread to the edges. Add a few strawberry slices on top if you like.

Chill and set: Refrigerate at least 1 hour before slicing. Lift the slab out using the parchment and cut into 12–16 bars with a sharp knife, wiping the blade between cuts.