1. Preheat the oven to 170ºC (150ºC Fan) and line two large baking trays with parchment paper.
2. To make the biscuits, place the butter and Lyle’s Golden Syrup into a small saucepan and heat gently until the butter is fully melted. Remove from the heat, add the milk, and set aside for the moment. In a medium bowl, mix together the flour, sugar, bicarbonate of soda, salt and spices. Add the egg yolk the dry goods, pour in the butter mixture and use a wooden spoon to mix and form a dough. Refrigerate for a minimum of 30 minutes before using.
3. To form into cookies, roll into small balls about 12-15g in size (I know this is a small size cookie and rolling into balls weighed out to such a specific amount will take a while, but you’ll be left with the cutest cookies which will make fabulous gifts). Roll the cookie dough balls into demerara sugar and place onto the prepared baking trays set a couple of inches apart. Bake in the preheated oven for about 25 minutes or until the cookies have spread out and firmed around the edges. Allow the cookies to cool fully on the baking trays.
4. For the filling, beat the butter in a large bowl, using an electric mixer, until soft and creamy. Slowly add in the icing sugar, a little at a time, beating to form a light and fluffy buttercream. Add in the vanilla, golden syrup and salt and beat briefly to combine.
5. To assemble, spread half of the cookies with a small amount of buttercream and sandwich together with a second cookie.