Preheat oven to 350°.
Combine flour, baking powder, soda, salt, sugar, cocoa, and cinnamon in a large bowl.
In a small bowl whisk together the buttermilk, eggs, butter, and vanilla.
Slowly add the buttermilk mixture to the flour mixture, stirring until just combined.
Pour the batter into a greased 8" square baking pan and bake for 30 minutes or until wooden pick inserted into the center of the cake comes out clean. Let cool completely before serving.