To a large bowl, add and combine the flour, ground ginger, cinnamon, mixed spice, bicarbonate soda and salt then mix and set aside.
In a stand mixer bowl fitted with a paddle attachment, cream together the butter and sugar until light and fluffy. You can use a hand mixer for this step.
Mix in the golden syrup and black treacle then gradually add and mix in the dry ingredients set aside earlier. Add in the milk and continue to mix until well combined.
Lightly flour a clean surface and scrape the dough onto the surface. Mould the dough into a disc shape then dough divide it into 4 equal pieces. Wrap each piece in cling film then refrigerate for 30 minutes.
Preheat the oven to gas mark 4/350°F/180°C/160oC fan then remove all 4 pieces of dough from the fridge. Roll a piece of dough out to around 1mm thick or as thin as you can without tearing the dough.
If the dough begins to crack when rolling, it means it is too cold. If this starts to happen then bring the dough back together and reshape it into a disc shape then roll it out again. The warmth from your hands should soften the dough.
Cut your biscuit shapes using a cookie cutter (see recipe tips above) then use a spatula to transfer them to a tray lined with parchment paper. There should be some space between each biscuit.
Repeat the previous process with the 3 remaining pieces of the dough and feel free to gather the scraps and roll them out again to make more biscuits. If you don’t have space to bake all the biscuits then place the leftover cut biscuits on a plate lined with parchment paper and refrigerate to bake later.
Bake the ginger thins in the centre of the oven for 11 - 12 minutes. Allow the ginger thins to cool on the baking tray for 10 minutes then transfer them to a cooling rack to completely cool.