These turned out just like I was imagining them! The secret is a little tool sold at Bed Bath and Beyond (and probably other places with baking tools.) See the pictures:
1/2 c. coconut oil
1 1/2 c. granulated sugar
3 eggs or egg replacer equivalent to 3 eggs (4 1/2 tsp Ener-G egg replacer and 6 T water or 3 T. ground golden flaxseed and 9 T. water- I prefer flaxseed)
2 tsp vanilla
1 c. brown rice flour
3/4 c. potato starch
1/2 c. tapioca starch
1/2 tsp xanthan gum
1 tsp baking powder
1 tsp baking soda
1/2 tsp salt
1 c. milk (I used coconut milk from a carton)
1/4 to 1/3 c. cocoa powder
Melt coconut oil and stir together with sugar in a large bowl. Add “eggs” and vanilla and stir well.
In a separate bowl, combine dry ingredients except the cocoa powder.
Add the dry ingredients and the milk to the wet ingredients, beginning and ending with the dry ingredients. I usually give it a quick mix with a mixer just to make sure all those gluten free ingredients are incorporated (about 1-2 minutes). The last thing you want is a mouthful of unblended rice flour. Separate out half the batter into a different mixing bowl and add 1/4-1/3 c. cocoa powder to one of the bowls. Mix thoroughly.
Place muffin liners into muffin tin. Stand the divider in one of the cups and put vanilla cake in one side and chocolate in the other. I found that about 1 1/2 to 2 T. worked well on each side. Fill all liners until batter is gone. If you run out of one, you can just use it to make a single-flavored cupcake. Bake in preheated oven at 350 degrees for about 18-19 minutes until cake springs back when you touch it.
Cool on a cooling rack and then top with frosting.
Final product when topped with my Milk Chocolate Frosting (posting in 2 days, so come back again!):