Grease and line with baking paper one 20cm round springform baking pan.
In a small saucepan over medium low heat, add butter and dark chocolate. Stir until fully mixed and smooth. Remove from heat and set aside to cool for 10 minutes.
In a large bowl, add self-raising flour, caster sugar, eggs, orange zest, orange juice and orange liquer. Stir to mix through.
Add chocolate mixture and using an electric mixer, beat for about 2 minutes until fully combined and smooth.
Pour mixture into baking pan, place in oven and bake for about 50 minutes or until a skewer inserted into the centre of the cake comes out clean. Remove pan from oven, let cool for about 10 minutes before turning out onto a wire rack to cool completely.
Ganache
Add dark chocolate, orange zest and pouring cream to a heat-proof bowl.
Place bowl over a saucepan of simmering water. Stir mixture until fully combined and smooth. Remove from heat and set aside to cool.
Once ganache has cooled to a spreadable texture, ice the top and side of the cake with ganache.