Heat oven to 180C/fan 160C/gas 4. Butter and line the base of a 1.2-litre loaf tin. Put all the cake ingredients into a bowl and beat with a hand whisk or wooden spoon for 3-5 mins, until light and fluffy. Spoon the mix into the tin and level the top.
Bake for 40-50 mins, until golden brown and firm to the touch. Meanwhile, heat the orange juice and sugar gently in a small pan, stirring until dissolved. When the cake is cooked, remove it from the oven and spoon over the orange mix. Leave to cool in the tin, then remove and cool completely on a wire rack.
Break up the chocolate and melt over a pan of simmering water or in the microwave on Medium for 1-2 mins. Drizzle over the cake and leave to set.