Prepare and bake the white cake mix according to the package directions in a 9x13-inch pan. Let the cake cool completely.

In a large bowl, dissolve the gelatin in the boiling water, stirring until fully dissolved. Stir in the cold water and, if desired, a few drops of blue food coloring for a deeper blue color.

Using the handle of a wooden spoon or a skewer, poke holes evenly all over the cooled cake, going about three-quarters of the way down.

Slowly pour the gelatin mixture over the cake, letting it seep into the holes. Cover and refrigerate for at least 4 hours, or until the gelatin is fully set.

In a chilled bowl, whip the heavy cream with the powdered sugar and vanilla extract until soft peaks form.

Spread the whipped cream over the chilled cake. Garnish with sprinkles or fresh berries, if desired.

Slice and serve cold.