Preheat oven to 350º. Spray or grease a 13x9 inch baking pan well.

Prepare cake according to the package directions. Pour into prepared baking dish and bake for 22-25 minutes, until it springs back when you gently tough the top, or a tester comes out mostly clean. Remove from the oven and let cool for about an hour.

Once the cake has mostly cooled, you want to poke holes all over the top. You can use the handle of a wooden spoon, and knife, or a chopstick works perfectly.

Start by mixing red jello with 1 cup of boiling water, whisking well until the jello has dissolved. Stir in 1/2 cup of cold water. Then using a straw or a meat injector syringe (see blog post for tips), fill half of the holes with red jello. Repeat using the blue jello.

Prepare the whipped cream by beating heavy cream to stiff peaks. Add in the powdered sugar and vanilla and beat until well combined. Spread over the cake. Top with festive sprinkles if desired.

Let the cake sit in the fridge for 2-3 hours for the jello to set up and chill. Then slice to serve.