Preheat oven to 325 degrees F. Spray a 9x13 pan lightly with non-stick spray.
Beat together the egg whites and cream of tartar until stiff peaks form. Set aside.
Mix egg yolks, orange juice, vanilla, oil and orange zest together until well combined. Add in dry ingredients and mix for 2 minutes on medium speed. Fold in egg whites, then transfer to 9x13 pan and bake for 30 minutes.
Cool cake completely.
To make the Sweet Cream Topping: Put orange juice in a small bowl. Sprinkle gelatin on top and lightly stir. Mixture will be clumpy. Microwave for 25 seconds, until mixture is hot and begins to boil. Stir until smooth. Beat heavy whipping cream in a mixer until it begins to thicken. Add in powdered sugar and orange juice mixture. Continue beating until soft peaks form. Set aside about 1 cup of cream for topping, if desired.
Spread remaining cream over cake evenly. Pipe 20-24 small swirls of cream onto top of cake. Top each swirl with a mandarin orange slice. Cut cake into squares and serve. Cover and chill remaining cake in the fridge.