My inspiration pic was this gorgeous nebula, the Eagle Nebula, photographed by Sid Leach, who has lots of other absolutely stunning pictures on his site that I highly recommend checking out. You may even find a nebula you like better for your cake inspiration!
And here's how the cake turned out:
What you need
Your favorite white cake recipe (minus a 1/4 cup of flour). I used this one from Joy of Baking, one of my favorite recipe blogs!
1/4 cup dark cocoa powder
Food coloring (I used red, yellow, blue, and green liquid food coloring, but I'm sure gel food coloring would work too, and black food coloring would probably be a good addition, especially if you want your cake to be darker than mine!)
Sprinkles (I recommend star shaped ones, and I also used silver ones!)
What you do
First, preheat your oven and make the cake batter according to your recipe, but leave out 1/2 cup of flour.
Separate the batter evenly into two bowls.
Add 1/4 cup flour to the batter that will be vanilla
Add 1/4 cup dark cocoa powder to the batter that will be chocolate
Take a few 1/4 to 1/2 cup portions of batter from the vanilla half and place in separate bowls.
Add blue food coloring to the main vanilla batter (or whatever color you want the main part of your nebula to be!), and color the smaller portions as desired. I did purple, lime, and a tiny bit of orange. I also added a few drops of blue and red to the chocolate portion.
Line round cake pans with parchment paper and spray with cooking spray
Drop batter a bit at a time, alternating colors, like you would for a zebra cake. I did about 1/3 cup chocolate and blue, and a few spoonfuls of the other colors at a time. I mostly dropped each bit right on top of the part below, but for the accent colors like orange I did just put them out on the edge randomly a couple times.
Shake the pan a little to level out the batter and use a knife to swirl the batter. I made one big swirl and then a couple side swirls on mine. Here's how they looked after swirling:
Bake according to recipe directions. For the recipe I used, that was 30 minutes at 350 degrees. This is how they looked fresh out of the oven:
To frost, I used fudgey dark chocolate buttercream frosting between the two layers and on top. I recommend adding a generous portion of star sprinkles to the middle layer--this time I put most of my star sprinkles directly in the batter and they were no longer recognizable after baking. Then frost the top as desired, and enjoy!