Preheat the oven to 180˚C/350˚F
Line a 23 x 13cm loaf tin with baking parchment. *
Mix the flour, salt, cocoa powder, sugar and orange zest together.
In a small saucepan over a medium heat, melt the margarine, treacle and milk. Once melted, add the bicarbonate of soda and the vanilla essence.
Pour the liquid ingredients into the dry and mix well.
Spoon into the prepared loaf tin, sprinkle with the chocolate chunks, and bake for 25-30 minutes, or until a skewer inserted into the middle comes out clean.
Once the cake is out of the oven, make the syrup: place the sugar and orange juice in a small saucepan and heat over a medium heat until the sugar is dissolved.
Prick the warm cake all over with a skewer or fork, and then pour over the drizzle. Leave in tin until completely cool, then remove and serve.
Will keep in an airtight container for 3-4 days, or freeze for up to one month.