Make the filling first. In a large bowl or stand mixer, add 6 tablespoons softened butter. Beat well until it is creamy and smooth. Add 3/4 cup packed brown sugar and 1 and 1/2 tablespoons cinnamon. Beat well, scraping sides down, until completely smooth. Line a baking sheet or large platter with parchment paper or a silicone mat. Clear out space in your freezer, if possible.Use a spoon to create small balls of filling, about the size of a marble. Line them up on the prepared pan, you should have quite a few (see photos). Place in the freezer if you can fit it in. Freeze for 20 minutes until hardened. If you only have room in the fridge, you will have to wait about an hour or two until they are firm.
Preheat the oven to 350 degrees and prepare a couple more baking sheets with parchment paper, or line with a silicone mat. (You can use the pan in the freezer to bake later, too)
Make the dough. In the same large bowl or stand mixer (no need to wash it), beat 1 cup butter for a couple minutes until smooth, scraping the sides and bottom a few times in between.
Add 1 and 2/3 cup white sugar. Beat for 2 minutes, taking the time to scrape the bottom and sides of the bowl in between. Your butter and sugar should be light and fluffy with no chunks at all.
Add 2 eggs and 2 teaspoons vanilla. Beat well, scraping the sides and bottom of the bowl. Make sure you beat it long enough that it becomes smooth and homogenous.
Add 3 and 1/2 cups flour but don't mix it in yet (make sure you spoon and level your flour).
Use a small spoon (I use my teaspoon) to stir in 1 teaspoon baking soda, 3/4 teaspoon kosher salt, and 1 and 1/2 teaspoons cream of tartar into the flour. Gently beat the flour mixture into the butter mixture. Don't overdo it. There should still be flour streaks when you stop your mixer. Use a spatula to scrape down the edges of the bowl.
Continue beating just a few more seconds until all the flour streaks are gone. Do not over mix! You want to make sure all the ingredients are combined, but once that is done, stop mixing. Over mixing dough = tough cookies.
Mix in. Once your small balls of cinnamon filling are frozen and firm, you can add them to the dough. Do not use a stand mixer or hand mixer. Instead, fold them in with a wooden spoon or a spatula.
Use a large cookie scoop or a spoon to shape the dough. You want dough balls that are about 2 inches across. I used this cookie scoop. and overfilled it slightly for each scoop. See photos.
Roll the cookies. In a small or medium bowl, add 1/3 cup sugar and 1 and 1/2 tablespoons cinnamon. Stir together. Roll the shaped cookies in the cinnamon-sugar to coat.
Place cookie dough balls on the prepared baking sheet with about 2 inches in between them. I can fit 12 cookies on an 11x17 baking sheet. But as you can see from the photos, they kind of run into each other. You can fix this with the spooning technique, but if you want to play it safe add 8 cookies to a pan.
Bake the cookies at 350 for about 11-12 minutes. These cookies are not going to look done when you take them out of the oven because the cinnamon filling will stay shiny. The edges of the cookies that DON'T have cinnamon filling pooling out the side should be firm but not golden. The top (white) part of the cookies should be matte (not shiny), at least on the outer edges. The cinnamon filling will be shiny and may or may not be spreading quite a bit. You might think these cookies are going to flatten out and crisp up and fail, but never fear, we have a plan:
Shape the cookies. Immediately after taking the cookies out of the oven (like seriously, within 30-60 seconds before the edges crisp), use a couple spoons to push the edges of the cookies toward their centers. Shape them back into the circle that they were meant to be. See photos! Scrape up all that oozing cinnamon sugar goodness and pack it into the edge of the cookies, pushing the cookies in on themselves. This makes the cookies thicker, chewier, and solves the problem of the cinnamon filling spreading everywhere.
Let the cookies set up on the pan for at least 5 minutes. Remove the cookies to a wire rack. Then eat one as fast as you can with a glass of milk! Cinnamon roll blissss!! Don't forget to try out my regular Snickerdoodle recipe sometime, it's almost the same recipe but instead of the cinnamon filling, it gets double dipped in cinnamon-sugar before and after baking. They are perfectly soft and chewy!
Freezer instructions: You can freeze this dough and bake later! I like to shape the dough into balls, roll them in cinnamon sugar, and store them in a ziplock bag. They will keep in the freezer for up to 3 months. I never thaw cookie dough before baking. Just bake straight from frozen and add a couple minutes to the bake time. Voila!