Adjust the oven rack to the middle position and pre-heat the oven to 350ºF (180ºC). Lightly butter a 9-inch (23cm) round cake pan and line its bottom with a round of parchment/baking paper.
Tip: Don't use a springform pan, as some of the caramel could leak out during baking.
In a saucepan, combine the light brown sugar, golden syrup, heavy/double cream, cinnamon and salt. Cook over medium-high heat, with frequent stirring, until the mixture comes to a boil, and then cook for 1 minute longer.
Add the chopped pecans and stir well until they’re evenly distributed.
Pour the caramel pecan mixture into the prepared cake pan and spread it out into a fairly even layer. Set it aside to cool for about 10 minutes while you prepare the cake batter.
Buttermilk cake:
In a large bowl, whisk together the light brown sugar, buttermilk, eggs, oil and vanilla until well combined.
In a separate bowl, whisk together the gluten free flour blend, baking powder, xanthan gum and salt.
Add the dry ingredients to the wet and whisk everything together into a smooth, fairly runny cake batter with no flour clumps.
Pour the batter on top of the caramel pecan topping and smooth it out into an even layer.
Bake at 350ºF (180ºC) for about 38-40 minutes or until the cake is golden brown on top, well risen and an inserted toothpick or cake tester comes out clean. If the cake starts browning too much or too quickly, cover it with a sheet of aluminium foil (shiny side up) and continue baking until done.
Inverting & serving the upside down cake:
Allow the cake to cool for 10-15 minutes in the cake pan, then carefully invert it onto a large plate while it's still hot. (Run a knife or a small offset spatula along the edges of the cake to loosen it from the pan if necessary.)
Some of the caramel pecan topping may stick to the pan. Don't panic if that happens, just scrape it off with a silicone spatula and arrange it on top of the cake.
Serve the cake either warm or cooled completely to room temperature.