Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper.
In a mixing bowl, cream together the butter and brown sugar until smooth.
Beat in the egg and vanilla extract.
In a separate bowl, combine the all-purpose flour, whey protein powder, cream of tartar, baking soda, salt, and 1 teaspoon of cinnamon.
Gradually blend the dry mixture into the wet ingredients.
In a small dish, mix the granulated sugar with the remaining 1 teaspoon of cinnamon.
Roll the cookie dough into balls and then roll them in the cinnamon-sugar mixture.
Place them on the prepared baking sheet.
Flatten each cookie using the bottom of a glass.
Bake in the preheated oven for 8-10 minutes, or until the edges are golden brown.
Allow the Chewy High Protein Snickerdoodle Cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.