Preheat oven to 180 C / Fan 160 C / Gas 4 and line a square cake tin with baking parchment, leaving some paper hanging from 2 of the edges, so it is easier to remove from the tin once cooked.
In a bowl using an electric mixer or hand whisk on a medium speed, lightly beat together the sugar, golden syrup and melted butter until well combined. Add the eggs and vanilla and continue mixing until well combined.
In a separate bowl, sieve the flour, cocoa powder and baking powder, and add cinnamon and salt. Combine with sugar mixture, adding a bit at a time, alternating with milk until all ingredients are well combined. Finally, fold in Oreo® pieces and white chocolate and spoon into the prepared tin.
Bake in the oven for 35 to 45 minutes or until a skewer inserted comes out clean. Remove from the oven and place on a cooling rack in the tin and leave to cool.
Once cooled, place on a chopping board, remove parchment from the sides and slice into the desired shape. Enjoy x