Preheat your oven to the temperature the boxed cake mix calls for. Spray a 9x13 baking dish with nonstick spray and set aside.

Prepare the cake batter in a medium bowl following the box directions (eggs, oil, water). When fully combined, transfer equal portions into four separate bowls.

Add a few drops of a different food coloring to each bowl (pink, blue, green, yellow). Stir gently and add more drops if you want deeper pastels.

Using a spoon, drop small dollops of each colored batter across the greased pan. Work quickly as the batter will spread.

Tap the bottom of the pan lightly on the counter to level.

Drag a toothpick through a few spots to create subtle marbling. Do not over-swirl; you want distinct pastel pockets.

Bake for the time the cake mix package specifies. Remove from the oven and let cool for about 10 minutes in the pan.

While still warm, poke holes across the surface about 1 inch apart using the end of a wooden spoon or a wide straw. Cool completely to room temperature.

In a large bowl, whisk the two boxes of instant white chocolate pudding with 3.5 cups cold milk until smooth and thickened. Let it sit for a minute if needed.

Pour the pudding evenly over the cooled cake, using a spatula to help press pudding into the holes so it fills the pockets.

Refrigerate the cake for 30–40 minutes, or until the pudding has set.

In another bowl, beat the heavy cream with vanilla until soft peaks form. Add powdered sugar and continue beating until stiff peaks form.

Spread the whipped cream over the set pudding layer and add sprinkles if desired. Slice and serve chilled.