Preheat the oven to 180° C/160° C fan/Gas mark 4 (350F). Lightly grease 2x20cm/8inch round baking pans.
Stir the lemon juice (or vinegar) into the milk and set aside to thicken and 'curdle' slightly into buttermilk.
In a pan over medium heat, melt the margarine, syrup and coffee granules together. Set aside to cool slightly.
Sieve the flour, cocoa, sugar, baking powder and bicarbonate of soda (baking soda) into a large mixing bowl and whisk together.
Pour the milk and melted margarine mixture over the flour mixture and stir well until it becomes a smooth batter.
Divide the mixture between the two prepared pans and bake for 30-35 minutes or until an inserted skewer comes out clean.
Allow the cakes to cool in the tins for 5 minutes, then turn out onto a wire rack to cool completely.
Meanwhile, to make the icing beat together all ingredients until smooth.
Once the cakes are completely cold, sandwich together with half of the icing. Spread the remaining icing to the top of the cake.