Preheat the oven according to the cake mix package directions, usually to 350°F. Grease a 9 x 13-inch baking pan
Prepare the cake batter according to the package directions. Gently fold in 1 cup of Fruity Pebbles
Pour the batter into the prepared pan and bake until the center is set and a toothpick comes out clean or with a few moist crumbs.
While the cake bakes, combine 1 cup whole milk and 1/2 cup Fruity Pebbles. Let the cereal infuse the milk for about 20 minutes
Strain the cereal from the milk and stir the infused milk together with the sweetened condensed milk.
Let the baked cake cool for about 15 to 20 minutes. Use the handle of a wooden spoon to make evenly spaced holes across the cake.
Slowly pour the cereal milk mixture over the cake, allowing it to absorb into the holes
Cover and refrigerate for at least 2 hours, preferably overnight.
Beat the cold heavy whipping cream, powdered sugar, and vanilla until soft to medium peaks form
Spread the whipped cream evenly over the chilled cake.
Sprinkle Fruity Pebbles over the top just before serving.
Slice into squares and serve chilled
