Preheat oven to 350° F (176° C). Spray two 9" cake pans with baking spray and set aside.
Sift flour with cocoa, baking powder, salt and baking soda. Set aside.
Cream butter, brown sugar, food coloring and vanilla, beating until well aerated. Add starter, eggs and buttermilk, then mix until well incorporated.
Create a well in the flour mixture and slowly add the starter mixture, mixing until combined after each addition.
Pour into prepared pans to ⅔ full. Bake for 35 minutes or until a toothpick inserted in the center of the bread comes out clean.
Allow to cool. If making a double-layer cake, gently cut off the top layer of each cake with a serrated knife to create a smooth top. Frost the top of one of the cakes and then turn over the other cake and place it on top of the frosted cake, so that the cut areas are in the center of the cake. Frost the top of the cake or make another batch of frosting if you'd like to frost the whole cake.
If you're noticing crumbs in your frosting, just frost a thin layer over the cake and place it in the fridge to set. After 30-45 minutes, remove the cake from the fridge and frost a fresh layer over the old one.