In a glass mixing bowl, microwave butter in 30-second intervals until it is melted.
Add brown and white sugar into the bowl and use a hand mixer to beat it together until all of the butter is absorbed by the sugar.
Add egg and vanilla extract to the mixing bowl and beat again.
Pour in the flour, salt, ground cinnamon, baking powder, baking soda, and cream of tartar to the bowl. Mix until all of the dry ingredients are absorbed.
Use a rubber spatula to fold in the oats - do not use a hand mixer here as you can ‘burn’ the oats and the cookies will spread.
Then, preheat the oven to 350°F and chill the cookie dough in the fridge for 30 minutes.
Use a cookie scooper to scoop the dough onto a parchment lined baking sheet. This recipe makes 18 cookies. You’ll likely need two cookie sheets or to bake two batches.
Bake cookies for 10-12 minutes or until the outsides of the cookie turn golden brown.
Remove from the oven and place onto a wire rack (or slide the parchment paper off of the baking tray with the cookies and let them cool off the pan).