In the bowl of a stand mixer (or using an electric beater), combine the flour, brown sugar, baking soda, cinnamon, ginger, mixed spice and salt on a low speed for about 20 seconds. Add the softened, cubed butter and mix at a medium-low speed until the mixture comes together into a crumbly texture.

Reduce the speed to low and, with the mixer running, slowly add the golden syrup and milk. Mix until the dough is evenly combined, about 30 seconds on a medium speed. 

Scrape the dough onto your kitchen bench; divide it in half. Shape the dough into two round discs and fully cover in glad wrap. Place in the fridge for at least 2 hours or overnight. 

Preheat the oven to 180C (350F). Line the two baking trays with baking paper, and place the caster sugar and icing sugar into seperate bowls. Take the cookie dough out of the fridge. Roll the dough into medium sized bowls, being careful not to overwork the dough. Roll each ball in the caster sugar and icing sugar to evenly coat, adding more sugar to the bowl if needed. Place the coated dough balls on the prepared baking sheet with a few centimetres between each. 

Bake the cookies for about 12 minutes, or until golden, making sure you don't overbake. Remove from the oven and allow to cool for about 2 minutes before transferring to a wire wrack to cool completely.  Store in an airtight container at room temperature for up to a week.