Every chocolate cake and buttercream I try to make from scratch is always way to light brown and the taste is never a yummy chocolate taste
5 lbs sugar
3 lbs cake fl
1 qt vegetable oil
1 qt warm water
1 1/2 oz baking soda
1/4 oz baking powder
1 1/4 lbs cocoa powder
Sift all the dry together except the cocoa powder. Put all the wet together. Then add the wet ingredients to the dry and mix until incorporated. Then add the cocoa and mix. It comes out like a very cakey brownie.
Hope this is dark enough for you!
Use Hershey's dark cocoa or King Arthur's black cocoa in your recipes. I usually use a mixture of regular cocoa plus some of the darker.
Experiment with both your cake and buttercream recipes. Add a little more cocoa and a bit more liquid and see how it comes out. (Without seeing your recipe, I'm taking a wild guess and suggest starting with two tablespoons cocoa and one tablespoon liquid.)
When I make chocolate buttercream, I never use a recipe, but start with my vanilla buttercream. I remove as much vanilla as I need to ice my cakes and then add enough cocoa and extra milk until it looks dark enough.
I use dark cocoa powder and hot coffee in my recipe and my cake comes out almost black. The cocoa powder plays a role for my cake, once the store ran out of the higher grade cocoa powder I used to use, and I bought the lower grade which was available, and I got a lighter colored cake with almost no cocoa aroma.
I make dark choc BC the way MimiFix explained above.
If you want really dark chocolate frosting, try ganache instead of buttercream. Then you'll definitely get the intense chocolate flavor you're looking for.
How about using ganache in your buttercream. I like it better than just adding cocoa. For the cake I would use the king arthur black cocoa. It may be exactly what your looking for. HTH!