GUAVA GEL: Prepare the guava gel while the oven is preheating. In a medium sauce pan, bring guava juice and sugar to a boil. make a thick liquid out of the cornstarch and a small amount of water, approximately 3 tablespoons or more if needed. remove guava juice mixture from heat and whisk in the cornstarch paste. Return to heat and bring back to a boil. Boil for one minute stirring frequently. Remove from heat, add butter and stir to combine. Cool on the counter while cake is baking, then in the fridge. Once cooled, stir in red food coloring until gel is the color you desire.
CAKE: Heat oven to 350 degrees. Mix the cake according to package directions. Stir in food coloring until it's the color you want. Bake in a 9 x 13 pan according to package directions.
CREAM CHEESE TOPPING: In a medium bowl, beat softened cream cheese until light and fluffy. Add sugar and vanilla and beat. In a separate bowl, beat the whipping cream until thickened, add 2 tablespoons of sugar. Continue beating until stiff peaks; do not overbeat. With a spatula, slowly fold in the whipped cream to the cream cheese. Refrigerate until ready to use.
ASSEMBLY: Thickly ice the cake with the cream cheese mixture. Pour the guava gel on top of the cream cheese mixture and spread evenly, being careful not to mix it into the cream cheese icing. Refrigerate until ready to serve.