Preheat the oven to 350F. Butter or spray a 9-10 inch cake pan & line with parchment paper *lining the pan is important so cake can be removed easily*.
In a small bowl whisk together cocoa powder, baking powder, flour & salt; set aside.
Put butter & dark chocolate in a small pot. Heat over medium low heat, stirring until smooth.
Transfer the melted chocolate mixture into a large mixing bowl; let cool for 5 minutes. Whisk in oil, sugar, vanilla extract & greek yogurt until combined.
Add eggs & mix well.
Add dry ingredients into the bowl & whisk until smooth.
Pour it into the prepared cake pan & bake 40-50 minutes or until a toothpick inserted to the center comes out clean.
Allow the cake to cool in the pan fully before removing from the pan to a cooling rack.
When the cake is completely cool, make the frosting and ice just the top of the cake using a swirling pattern.
Notes
While the frosting is divine, if you want a super simple cake just skip the icing and top the cake with whipped cream and fresh berries.
If you want to make this cake ahead of time, once completely cooled after it bakes, wrap tightly in foil or plastic wrap and store at room temperature. When ready, unwrap the cake, frost it and serve.
Once frosted, cover the cake with a 'cake keeper' or an overturned large bowl - this will help keep out dust etc. You can store this cake for 3 days at room temperature.