Prepare the white cake as per the package instructions in a 9x13 pan. Allow to cool for 10 minutes and then poke all over with a chopstick. Set aside.
Combine each package of gelatin in a separate bowl with ¾ cup of boiling water. Stir until completely dissolved and then add ½ cup ice water. Use a measuring cup to carefully pour each mixture into alternating straight lines across the cake. Pour the lines slowly, allowing the cake to absorb the mixture completely. Chill for at least 4 hours.
In a cold bowl, using a hand mixer on medium speed, beat the heavy cream, sour cream, confectioners sugar and vanilla extract until medium peaks. Spread the whipped cream evenly over the cake. Top with sprinkles. Cut and serve immediately.
TIP: The sour cream stabilizes the whipped cream to stay whipped for longer.