Zest both of the of the oranges. Squeeze the juice from both oranges and strain into a small saucepan. Add the sugar and half the zest and stir to dissolve. Add the egg yolks and mix well and place over a low heat stirring constantly. Cook for 10 to 15 minutes or until the curd coats the back of the spoon. Add the butter and stir until melted and smooth. Pour into a sterilised jar and refrigerate for at least 3 hours.
In the bowl of a stand mixer combine the butter, sugar, remaining orange zest and golden syrup and bet until pale and creamy, approximately 10 minutes. Add the sifted flour, cocoa and baking soda and mix until a smooth dough begins to form. Scrape down the sides of the bowl and mix again. Add the chocolate and mix until combined. Cover the bowl and refrigerate for 30 minutes, or until firm.
Preheat the oven to 180 celsius (350 Fahrenheit) and take tablespoons of the dough and roll into balls and place on a baking tray lined with baking paper. Flatten the balls slightly and bake for 10 to 12 minutes or until the tops of the bookies begin to crack. Remove from the oven and leave on the tray for 2 minutes to firm up before placing on wire wracks to cool.
Spread half the cookies with curd and top with the remaining cookies.
The filled cookies should be stored in an airtight container in the refrigerator for up to 4 days. Alternatively fill the cookies as needed. Unfilled cookies can be kept at room temperature in an airtight container for up to a week.