In the bowl of an electric mixer, beat the butter for a minute until soft and smooth.
Add the sugars, cream of tartar, baking soda, salt and cayenne pepper and beat until everything is combined.
Scrape down the sides of the bowl and mix in the eggs and vanilla.
Add the cocoa and flour a little at a time until everything is mixed in and there are no dry bits.
Pop the bowl in the fridge for about an hour for the dough to firm up.
Pre-heat the oven to 160C fan-forced and line two baking trays with baking paper.
Using about a teaspoon of dough at a time, roll the dough into balls and then roll in the cinnamon/sugar mixture.
Place the balls of dough on the tray leaving 3cm between each one.
Bake the snickerdoodles for 10-12 minutes or until they start to crack on the top.
Leave them to set on the trays for a minute and then transfer to a wire rack to cool.