Preparation: Preheat the oven to 165°C (325°F, Gas Mark 3). Generously butter an 8 x 3-inch deep round cake pan or a 9 x 2-inch round cake pan.
Pecan Layer: In a mixing bowl, combine chopped pecans, golden syrup, brown sugar, melted butter, lemon juice, and vanilla extract. Stir until well mixed and set aside.
Dry Ingredients for Cake: In a separate bowl, sift together the all-purpose flour, baking powder, baking soda, and salt. Set this mixture aside.
Cake Batter: In the bowl of a stand mixer fitted with the whisk attachment, whip the eggs for approximately one minute. Gradually add the sugar and continue to whip until the mixture is light and foamy. Slowly drizzle in the oil, followed by the vanilla extract and buttermilk, mixing until fully incorporated.
Combining Ingredients: Gradually fold the dry ingredients into the wet mixture using a spatula, scraping the sides and bottom of the bowl to ensure a uniform batter without overmixing.
Assembling the Cake: Pour the pecan mixture into the prepared cake pan, spreading it evenly. Carefully pour the cake batter over the pecan layer, ensuring it is evenly distributed. Tap the pan gently on the counter to remove any large air bubbles.
Baking: Place the cake in the preheated oven and bake for 35 to 45 minutes, or until a skewer inserted into the center of the cake comes out clean.
Inverting the Cake: Allow the cake to cool in the pan for 5 to 7 minutes. Then, place a plate or platter larger than the cake pan on top of the pan, and using a kitchen cloth or gloves, firmly hold the top and bottom and flip the pan in one swift motion to invert the cake onto the plate.
Serving: Serve the cake warm or at room temperature, ensuring the caramelized pecan layer is displayed prominently on top.