Preheat oven to 350F/180C/Gas 4. Grease an 8-inch round cake pan. Ideally, cut a piece of parchment paper to cover the bottom of the pan, and grease the paper.
Prep the orange zest. Only grate the bright orange part of the peel and not any of the white pith. Juice the oranges after they're zested.
In a large bowl sift together the flour, cocoa powder, baking soda and baking powder. Then, whisk in sugar and salt.
Mix together the oil, vanilla, orange juice and zest in a small bowl. Make a well in the centre of the dry ingredients and pour in the liquid ingredients. Whisk the cake batter together until just mixed (no large clumps of dry flour).
Pour batter into the greased cake tin. Tap the pan on the counter a couple times to get the batter flattened. Bake cake in oven for 33-35 mins. A toothpick inserted into the centre of the cake should come out clean when the cake is done.
Allow the cake to cool in the pan for 5 mins before carefully running a knife around the edge of the cake pan to loosen the cake. Turn out onto a cooling rack and allow to cool completely before frosting it.
MAKE THE FROSTING (needs to set in the fridge before using): Melt vegan butter or margarine in a medium sized bowl. Sift in icing sugar and cocoa powder. Stir in orange zest, orange juice and vanilla extract. Stir until the mixture looks smooth.
Cover the bowl or transfer to a container with an airtight lid. Allow frosting to chill in the fridge for at least 30 mins or until the cake is completely cool (frosting will harden as the butter chills).
Spread frosting across chilled cake. Get stuck in.