Prepare the crust: Line an 8x8-inch baking dish with parchment paper. Mix crushed graham crackers and melted butter until combined, then press firmly into the dish. Chill while making the filling.
Whip the cream: Using a hand mixer, whip the heavy cream until stiff peaks form. Set aside.
Make the filling: Beat cream cheese, powdered sugar, and vanilla extract until smooth. Gently fold in the whipped cream until fully combined.
Add mix-ins: Fold in the mini rainbow marshmallows and white chocolate chips until evenly distributed.
Assemble and chill: Spread the filling evenly over the crust, smooth the top, cover, and refrigerate for at least 4 hours until firm.
Slice and serve: Lift from the pan using parchment paper, cut into squares.