Start by preheating the oven to 180 degrees Celsius. Then line baking trays with baking paper. Place the butter, sugar and golden syrup in a bowl with an electric mixer. Beat until the mixture is pale and creamy. Add the eggs and beat until just combined, then transfer to a large bowl. Add the flour, cocoa powder, bicarbonate of soda, ginger and mixed spice and stir to combine. You'll need to use your hands to bring the dough together. Turn the dough onto a lightly floured surface and gently knead until smooth. Cover with plastic wrap and pop in the fridge for 30 minutes or until firm.
Next roll out the dough on a lightly floured surface to a three millimetre thick disc. Using a nine centimeter gingerbread cutter, cut shapes out of the dough and place on a line tray. Continue re-rolling the excess dough to make about 30 biscuits. Pop them in the oven and bake for 10 to 12 minutes or until the biscuits are just firm. Set aside on the trays to cool. To make the royal icing, whisk the egg whites and lemon juice in a medium bowl and whisk with a fork until slightly frothy.
Gradually add the icing sugar stirring well after each addition until a smooth paste forms. Place the icing sugar in a piping bag fitted with a one millimetre plain nozzle. Pipe over the biscuits and use the icing to attach the M&M's to decorate. Set aside for 30 minutes to set these chocolate gingerbread biscuits are a fun twist on the traditional classic.