I thought you only use baking soda when you use buttermilk so why is it baking soda not baking powder

To add to @Kristen Miglore's response,
the natural cocoa used in this recipe is not dutch-processed, so it is acidic. That's why the baking soda is used in this recipe. If this recipe had used dutch-processed cocoa then it would have needed the baking powder for leavening since DP cocoa is neutral and the baking soda (a base) would have no acid to react and create leavening for the cake.