In a large bowl, add butter, white sugar & brown sugar.
Using an electric mixer, beat until well combined and creamy.
Add egg & vanilla extract and beat until well combined and light & fluffy.
Add plain flour, bicarb soda, cream of tartar & salt and continue beating until just combined.
Using your hands form between 12 – 14 balls of dough. Each ball will be about 2 heaped tbsps of dough.
Place each dough ball on a baking tray lined with baking paper. Flatten each dough ball slightly.
Cover dough balls and baking tray with plastic wrap and place in refrigerator for 90 minutes for the dough to chill.
Preheat oven to 180°C.
In a small bowl add caster sugar and cinnamon and stir to mix thoroughly to create the cinnamon sugar.
Remove dough balls from refrigerator and roll each ball in the cinnamon sugar to fully coat.
Place each ball at least 5cm apart on the baking tray lined with baking paper. Use additional baking trays, if needed.
Bake in oven for 9 minutes. The cookies should flatten out and bet cooked around the edges and golden brown. This amount of time should produce nice soft centred cookies. Cook and extra minute or two to get firmer cookies.
Remove from oven and allow to cook on the baking tray. The cookies will be very soft when initially removed from the oven but will firm up as they cool.