Heat the oven to gas 4, 180°C, fan 160°C. Line two baking sheets with nonstick baking paper.
Using a hand mixer beat 100g butter and sugar together until combined, then beat in the golden syrup.
In a separate bowl sift the flour, bicarbonate of soda and 2 tbsp cocoa powder. Fold through the butter mixture then bring together to form a dough.
Lightly flour a clean surface then roll out the dough to 1/2 cm thick. Using a large knife cut the dough into 3cm x 6cm rectangles. Transfer to the baking sheets leaving space for each to spread a little. Using a skewer make 10 indents in a grid pattern half way into the dough then transfer to the oven for 10 minutes.
Leave to cool on the trays while you make the filling. Sift the icing sugar and 2 tbsp cocoa powder together. Add the remaining 75g (3oz) butter and vanilla and beat together with an electric hand mixer until soft and creamy. Stir through the milk.
When the biscuits are completely cooled use a knife or piping bag to cover one side with butter cream then sandwich with another biscuit.
These biscuits freeze really well so make an extra batch and freeze once baked. Defrost for 4 hours before icing.
See more Dessert Recipes