Melt the butter and dark chocolate together in a plastic bowl in the microwave then add in 360ml water

Add in the orange zest 

Sift the flour, cocoa powder, baking powder and bicarbonate of soda into a large bowl, stir in the sugar

Mix the ground flax and 45ml water to make a flax egg

Add in the flax eggs, crème fraiche and oil and then fold into the chocolate mixture

Bake in a lined round tin at 160°C for 20 minutes until the cake is springy

Decorate with melted chocolate and crystallised orange