Preheat the oven to 175 Celsius (350 degrees Fahrenheit) and butter 1 8-inch/20cm round cake tin and line with baking paper.
Sift the flour, cocoa, baking soda, baking powder, and salt into the bowl of an electric mixer fitted with a paddle attachment and add the sugar and mix on low speed until combined.
In another bowl, whisk together the buttermilk, oil, eggs,orange zest and vanilla. With the mixer on low speed, slowly add the wet ingredients to the dry.
With mixer still on low, add the orange juice and mix until just combined.
Pour the cake batter into two 20cm prepared cake tins and bake for 35 to 40 minutes or until a cake tester comes out clean.
Cool in the pan for 30 minutes, then turn out onto a cake rack and cool completely.