Preheat the oven to 180C fan (350F fan) or 200C non-fan (400F non-fan). Line a 20cm (8-inch) square baking tin with baking paper.
Place the vegan butter, sugar and golden syrup in a medium pot. Place on a medium heat on the stove and leave until the butter has melted, mixing every now and then.
Remove from the heat and add in the flour and oats. Stir until there are no floury patches remaining and all the oats are well-coated.
Add any 'mix ins' you want now (except chocolate chips as those will just melt), reserving a handful for the top for decoration, and stir together.
Tip the contents of the pot into the lined tin and press down to form an even layer. You can sprinkle your reserved handful of additional 'mix-ins' or, if you're using chocolate chips, sprinkle them on now. Press them into the flapjack mixture.
Bake for 20-25 minutes until bubbling with golden edges. They will seem very soft when they're hot so allow them to cool before slicing into 16 squares.
Store in an airtight container for up to 2 weeks or wrap and freeze for up to 2 months.