Can you recommend a cocoa powder that makes extremely dark brown cakes where the color is almost black?

There is a product called Black Cocoa Powder -- used to be available at King Arthur's, not sure if it still is. It is very bitter, you only use it as part of your cocoa. It adds a lot of darkness to the look and mystery to the taste. Also, for regular cocoa powder but a dark look, search for Black Magic Cake recipe; been around for decades. It is a wonderful, flavorful, moist chocolate cake (the one I make 90% of the time) with coffee and buttermilk as the liquids. It's very dark, but doesn't hold up to stacking very well.

I use Hershey's Special Dark or Droste's cocoa. Have never had a problem with my cakes coming out too light.

I like Dutch-processed cocoa for its dark colour, and King Arthur Flour also sells black cocoa, for really dark results.

You want a cocoa powder that is a good 24% fat. You can usually find one in a good store's bulk foods section. The grocery store varieties tend to be about half that.

I've made plenty of chocolate cakes with Hershey's Special Dark cocoa powder (dutch processed) and they come out dark and chocolatey. Easily available. I think the chocolatey-est cakes are made with cocoa powder by the way.

I use valhrona is very very dark and produces a cake that is almost black. Its available at many stores I'm sure but I get it at whole foods columbus circle.

I think you're looking for Dutch-processed cocoa. Cakes made with it tend to come out darker than those made with regular cocoa powder.