Hello! Here is a project my son and I did in February for his class. The kids were THRILLED with the results!

Step 1: Preparation

I used two boxes of white cake mix, and prepared according to the instructions. You could make your own cake batter; just be sure it's white, not yellow, so it will colour properly.

Step 2: Colouring the Batter

I divided the batter into six bowls, and added food colouring to achieve the desired rainbow shades (red, orange, yellow, green, blue, violet). ROYGB-V!

Step 3: Layering the Batter

I used two layers of cupcake papers - this helps cupcakes to keep their shape better (remove the outside layer after they cool).
We carefully deposited a teaspoon of each colour in the cups, beginning with violet, and working our way up the rainbow. When we were done filling the cups, we gently shook the cupcake pan back and forth to settle the batter a bit.

Step 4: Rainbowy Goodness

This is what the rainbow cupcakes look like!

Step 5: Making Pop Tart Bodies

I considered making butter cookies to represent the entire Nyan Cat, but that would have required a lot of fiddly cutting. When I saw these social tea biscuits in the grocery store, I knew they would be perfect. I iced them with pink icing, then we applied tiny red icing dots to look like Nyan Cat's pop tart body.

Step 6: Nyan Cat Head Cookies

I made Nyan Cat's head out of butter cookie dough. I used a tiny heart cutter, and just nipped off some edges to achieve a passable shape. My son wanted his to look more pixelated, so he cut quite a few by hand. When the cookies were cool, we iced them with thin white icing, let that dry, then used blue icing for features.

Step 7: Assembling the Cupcakes

Finally, we iced the cupcakes with dark blue icing, and applied the pop tart body on top. Then we used white icing to attach the heads, and to create some little cat legs below the pop tarts. They were a bit blobby, so after the icing had dried, I used a plastic toothpick to define the legs a bit.
This was an enormously fun project for my son, and the kids in his class went ballistic for the cupcakes.
It was time-consuming mainly due to the cooling and drying times between steps.
I made the cupcakes and cookie parts the night before I took them to school, and assembled them in the morning.
I hope you enjoy this as much as we did!