Melt the butter in a small pot and then add the syrup and sugar and heat.
When the butter is bubbling, add the bicarb (baking soda) and stir through and remove from the heat. It should bubble and form a fluffy mallow texture.
Pour the butter mixture into the dry ingredients and stir together by hand.
Using the back of a metal spoon, gently press the crunchie mixture into a greased or lined baking tray (aprox 30cm x 20cm or similar depending on how thick you like the crunchies. A thinner layer will make crisper crunchies and a thicker layer will make them chewier.
Once you have pressed the mixture down, pre-cut/ score the crunchies about halfway to 3/4 of the way through with a sharp knife. This makes cutting them once baked a bit easier. This is not as important if you like chewier crunchies.
Bake for 15 minutes at 180 C/350F then turn the oven down to 160C/325°f and bake for a further 10 minutes until golden brown. Keep a close eye as they can burn easily
Allow to cool in the pan before slicing into squares.
Notes
Store these crunchies in an airtight container for up to 4 weeks.