Preheat your oven to 160/140C Fan, and line a 9x13" Tin with parchment paper.
In a heatproof bowl, microwave the dark chocolate and butter until melted – mine takes two minutes!
Pour the coffee granules into 125ml boiling water and mix – add the coffee to the chocolate/butter mix and stir well till smooth!
In a separate large bowl add the plain flour, cocoa powder, baking powder, bicarbonate of soda, caster sugar, and light brown sugar and mix well so the ingredients are all mixed with each other well!
Mix the eggs with the buttermilk and orange extract – Add the chocolate mix and the egg mix to the dry ingredients and stir together, try not to over mix it – you don't need to use an electric mixer, just use a bowl and a spatula.
Pour the mixture into the tin and bake in the oven for 50-60 minutes (can take a little longer in some ovens) once baked leave to cool in the tin and remove afterwards, or leave for 10-15 minutes and cool on a wire rack for a softer cake.
For the Frosting
With an electric mixer with the paddle attachment, beat the butter until its very smooth and supple (couple of minutes)
Gradually add the icing sugar & cocoa powder alternatively with the evaporated milk until all is incorporated and add the orange extract – beat for a couple of minutes so its extra smooth & creamy.
The more evaporated milk you add the more runny it’ll be, and I have found about 100ml-125ml is perfect for me!
Decorate your cake how you like - I used sprinkles, orange zest and terry's chocolate orange!
This cake will last for 3 days at room temperature.