Preheat oven to 180 ° C and grease a 8" springform or ring mold cake tin with butter or oil.
I a mixing bowl sift together flour, cocoa powder, baking powder, baking soda and salt.
In another bowl beat together butter and sugar until creamy and soft.
Melt chocolate in double boiler and add it to the butter-sugar mixture. Stir in buttermilk and vanilla extract.
Combine the flour mixture into the wet mixture in two or three batches and mix gently until all the ingredients are mixed.
Transfer the batter into the prepared cake tin. Bake in preheated oven for about 30 minutes or until a cake tester inserted in the centre of the cake comes out clean.
Remove the cake from oven and let it sit in the cake tin for 10 minutes. Unmold the cake and let it cool.
Once cooled completely slice the cake into two layers and set aside.