Preheat the oven to 190ºc/170ºc fan and line a 9" square tin with parchment paper
Add the butter, sugar and syrup to a heavy based pan
Melt together on a low-medium heat, until the sugar has dissolved - make sure to not let this mixture boil
Pour in the oats and mix together. The oats should be fully covered in the butter/sugar mixture, without being sopping wet.
If using, mix through the raisins as well.
Press firmly and evenly into the lined tin. I use my spatula that I made them with first, and then use a metal spoon to press down as well.
Bake for 25-30 minutes, until golden brown. There may likely be a wobble to the mixture when it comes out the oven, but it should be evenly gold all over the top
Leave to cool fully in the tin, and then portion and enjoy
Notes
These flapjacks will last for 5-7+ days at room temperature, or in the fridge on a hot day.