Preheat the oven to 160 degrees celsius or 320F. Line the baking sheet with parchment paper.
In a large heavy bottomed sauce pan, add the butter, sugar and cream of tartar on medium heat.
Mix with a silicone spatula regularly, so that it melts completely.
Once its all dissolved, take the pan off the heat.
Add the vanilla and baking soda (This will bubble up).
Pour the caramel over the popcorn and mix well.
Spread the caramel coated popcorn on to the baking sheet.
Bake for about an hour mixing it every 15 minutes.
Let it cool and store in an airtight container.