For the sponge
1. Preheat the oven to 180 ºC/160 ºC fan/Gas 4/355 ºF and line an 8x8 inch (20x20 cm) square baking tin with baking paper.
2. Using a stand mixer with the paddle attachment or a hand mixer with the double beater attachments, cream the butter, sugar, Lyle’s Golden Syrup and vanilla extract together until pale and fluffy.
3. Add the eggs, one at a time, mixing well after each addition.
4. Sift together the flour, xanthan gum (optional), baking powder and salt.
5. Add one-third of the dry ingredients to the butter mixture and mix to combine. Add half the milk and mix to combine. Repeat with the remaining dry ingredients and milk, finishing with the addition of dry ingredients. Mix well until you get a smooth cake batter with no flour clumps.
6. Transfer the batter into the lined baking tin and bake at 180 ºC/160 ºC fan/Gas 4/355 ºF for about 1 hour or until risen, golden brown on top and around the edges, and an inserted toothpick comes out clean. If the top starts browning too quickly, cover with aluminium foil (shiny side up) and continue baking until done.
7. Once baked, cool in the baking tin for about 10 minutes, then remove from the tin and allow to cool completely on a wire cooling rack.