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 ¾ cup packed brown sugar and 1 and ½ 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. Place in the freezer if you can fit it in. Freeze for 20 minutes until hardened.

Preheat the oven to 350 degrees and prepare a couple more baking sheets with parchment paper, or line with a silicone mat.

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 ⅔ cup white sugar. Beat for 2 minutes, taking the time to scrape the bottom and sides of the bowl in between.

Add 2 eggs and 2 teaspoons vanilla. Beat well, scraping the sides and bottom of the bowl. Add 3 and ½ cups flour but don't mix it in yet. Use a small spoon to stir in 1 teaspoon baking soda, ¾ teaspoon kosher salt, and 1 and ½ teaspoons cream of tartar into the flour. Gently beat the flour mixture into the butter mixture. Don't overdo it.

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. 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. Bake the cookies at 350 for about 11-12 minutes.

Immediately after taking the cookies out of the oven, use a couple spoons to push the edges of the cookies toward their centers. Let the cookies set up on the pan for at least 5 minutes. Remove the cookies to a wire rack.