- Preheat the oven to 180C/350F and spray a 10 inch cast iron skillet with cooking spray. I used Wilton Bake Easy. 
- Place butter and sugar into a large bowl, or the bowl of your stand mixer and beat on medium-high speed until light and fluffy, around 2 minutes. 
- Add in the vanilla and eggs, scraping down the sides as necessary, and mix on medium speed until well incorporated and smooth, around 1 minute. 
- Add in the flour, cinnamon, cream of tartar, bicarb and salt and mix on low until the mixture comes together and is smooth, with no lumps of flour. 
- Spoon the mixture into your prepared skillet and smooth into an even layer with the back of your spoon; mixture will be pretty thick. 
- Place in the oven for 25-30 minutes until risen and an inserted skewer into the centre comes out clean. 
- Leave to cool in the skillet for at least 30 minutes. 
- Place 3tbsp sugar and 1/2tsp cinnamon into a small bowl and mix until well combined. Sprinkle the cinnamon sugar all over the top of the Snickerdoodle cake. 
- Top with vanilla ice cream and drizzle with caramel sauce, if desired. 
- Alternatively you can leave to cool completely and then slice into wedges. We loved this warm, but it was great the next day cool as well! 
- Snickerdoodle Skillet Cake will keep without the ice cream and caramel sauce in an airtight container, at room temperature for 4 days.