Preheat the oven to 350°F (180°C). Grease 2 9 x 13-inch (22 cm x 30 cm) baking pans with nonstick spray and line with parchment paper.
Make the blondies: In a large bowl, combine the brown sugar, melted butter, and eggs. Mix with an electric hand mixer on medium speed until combined. Add the yellow cake mix and mix on low speed until just combined. Pour the batter into a prepared baking pan and spread evenly.
Make the brownies: Pour the brownie batter into the remaining prepared baking pan and spread evenly.
Bake for 28–30 minutes, or until the blondies are golden brown and slightly crackly and a toothpick inserted in the center of the brownies comes out mostly clean with a few moist crumbs. Let cool completely.
Make the topping: Add the cold water and gelatin powder to a small bowl. Mix with a fork to combine. Set aside while you whip the cream.
In a large bowl, combine the cream cheese, granulated sugar, salt, and vanilla. Mix with an electric hand mixer on medium-high speed until smooth, about 3 minutes.
With the mixer running on low speed, slowly stream in 2½ cups (600 ml) of heavy cream. Once the cream has been incorporated, continue mixing on high speed for about 2 minutes, or until light and fluffy.
Microwave the remaining 3 tablespoons of heavy cream for 15 seconds, then pour on top of the gelatin mixture. Stir with a fork just until the gelatin dissolves.
Add the gelatin mixture to the whipped cream, then whip on high speed for 2 minutes.
Assemble the blondies: Pour half of the whipped cream mixture over the cooled blondies and spread evenly. Sprinkle the crushed fruit cereal over the cream, then top with the whole fruit cereal, pressing lightly into the cream. Cover with plastic wrap and refrigerate for at least 2 hours, or until ready to serve.
Assemble the brownies: Pour the remaining whipped cream over the cooled brownies and spread evenly. Sift the cocoa powder over the cream, then top with the peanut butter and chocolate-flavored cereal, pressing lightly into the cream. Cover with plastic wrap and refrigerate for at least 2 hours, or until ready to serve.
To serve, remove the chilled blondies and brownies from the refrigerator. Lift from the pan and onto a cutting board, then cut each into 12 pieces.