Set the oven to 350°F and coat a 9x13-inch baking dish lightly with baking spray. A thin coating across the bottom and sides helps release the baked brownie.

Stir the strawberry cake mix, egg, melted butter, and heavy cream together in a medium mixing bowl. The finished batter should look uniform and thick, with no dry pockets of mix.

Distribute the white chocolate chips through the batter with a spatula, then press the batter into an even layer in the prepared dish. Check the corners and center for roughly equal thickness so the brownie bakes evenly.

Bake for 18 to 23 minutes, check for doneness, and let the brownie cool to room temperature before continuing. Look for golden edges and a toothpick that comes out clean from the center.

Whip the room-temperature cream cheese with the powdered sugar in a large bowl using a hand or stand mixer. Stop when the mixture looks fluffy and smooth rather than lumpy.

Blend the sour cream and vanilla into the cream cheese mixture with the mixer. The frosting should look evenly creamy, without streaks of sour cream.

Use a spatula to fold in the whipped topping, then spread the frosting across the completely cooled brownie. Gentle folds preserve the fluffy texture, and the finished layer should cover the surface evenly.

Break down the vanilla sandwich cookies in a food processor using short pulses. Leave recognizable coarse pieces rather than processing everything into powder.

Pour the strawberry syrup over the cookie crumbs and use brief pulses to distribute it. Stop as soon as the crumble has an even, pale pink coating.

Scatter the strawberry crumble evenly over the frosting, then cut into squares to serve or refrigerate until serving. The topping should form a loose, textured layer rather than a tightly packed crust.