Line a baking tray with baking paper, set aside.
Heat the oil in a large saucepan on medium high heat. Put a few popcorn kernels into the oil and cover the saucepan.
Once the kernels pop, add the rest of the ½ cup of popcorn kernels in an even layer.
Cover, remove from the heat and count 30 seconds. This method I have learnt first heats the oil to the right temperature, then waiting 30 seconds brings all of the other kernels to a near-popping temperature so that when they are put back on the heat, they all pop at about the same time.
Return the pan to the heat. The popcorn should begin popping soon, and all at once. Once the popping starts vigorously, gently shake the pan by moving it back and forth over the burner. Try to keep the lid slightly ajar to let the steam from the popcorn release (the popcorn will be drier and crisper). Once the popping slows to several seconds between pops, remove the pan from the heat, remove the lid, and dump the popcorn immediately into a wide bowl.
Break up 3/4 of the popcorn into small bits, this will become the base of the brownie. Leave the remaining 1/4 as is in a separate bowl.
Now, on to the caramel. In the same saucepan, place sugar, golden syrup, butter, vanilla extract and heat on low to medium heat until sugar has dissolved.
Increase to medium heat (but watch it like a hawk, sugar burns easily) and cook without stirring for around 2 minutes (please note this is a reduced cooking time as it continues to cook in the oven).
Stir in baking soda and then add popcorn and stir furiously to coat kernels all over before the caramel starts to set.
Quickly press 3/4 of the broken up popcorn into a lined 33 x 25 x 5.5 cm pan.
Spread the remaining 1/4 out onto the lined baking tray and allow to cool and set.