Using a straw or the handle of a wooden spoon, poke holes all over the surface of the baked and cooled cake.
Prepare each box of Jell-o using 1 cup of boiling water + 1/2 cup of cold water. (This is 1/2 cup less cold water than what is called for on package instructions.)
Carefully pour or spoon the grape Jell-o over a portion of the cake. Take care to get the liquid into the holes.
Carefully pour or spoon the orange Jell-o over the rest of cake. Take care to get the liquid into the holes.
Refrigerate the cake for at least 3-4 hours or overnight - this allows the Jell-o to set up within the cake.
Frost the cake with the whipped vanilla frosting.
Just prior to serving, top with Halloween sugars and sprinkles.
Cut and serve!
Refrigerate any leftover cake