Chop the butter into chunks and place in a large mixing bowl. Beat using an electric mixer or wooden spoon for 1 minute. You want the butter to be slightly softer, broken up, but not completely smooth.
Add the light brown sugar and white caster sugar and beat together until just combined. Try not to over-mix at this stage otherwise, the butter will become too soft.
Mix in both of the eggs until combined.
In a separate bowl, stir the self-raising flour, plain flour, salt, baking powder and cinnamon together. Add to the butter mixture and mix until it comes together to form a thick cookie dough.
Add the white chocolate chips and use your hands to mix them into the dough.