Preheat oven to 350F (175C) and line several baking sheets with parchment paper. Set aside.

In a large mixing bowl (or the bowl of a stand mixer fitted with the paddle attachment) combine butter, peanut butter, and sugars and use an electric mixer to beat until creamy and well-combined.

Add egg, yolk, and vanilla extract and stir until well-combined.

In a separate, medium-sized mixing bowl, whisk together flour, cream of tartar, baking soda and salt.

Gradually (in 3 or 4 parts) stir the flour mixture into the peanut butter mixture, pausing periodically to scrape the sides and bottom of the bowl to ensure all ingredients are well-combined.

In a separate, small dish, prepare the cinnamon/sugar for rolling by whisking together the sugar and cinnamon.

Scoop dough by rounded 1 ½ Tablespoon-sized scoops (about 36g) and roll between your palms to form a smooth ball. Roll thoroughly through the cinnamon/sugar mixture and place on prepared baking sheet, spacing cookie dough balls at least 2” apart. Keep dough covered as it rests on the counter between batches.

Transfer baking sheet to center rack of 350F (175C) preheated oven and bake for 8-10 minutes. Do not overbake cookies, they may look slightly underdone in the centers and this is fine, they will continue to cook as they cool. Overbaked cookies will be dry and crumbly (if overdone, bottoms will be golden brown, too, this is a good indicator that they’re overbaked but of course it’s not possible to check this til they’ve cooled).

Allow cookies to cool on baking sheet for at least 10 minutes before carefully removing to a cooling rack to cool completely before enjoying.