First, make the cinnamon roll filling. In a large bowl using a hand mixer, or in a stand mixer using the paddle attachment, cream butter, brown sugar, cinnamon, and salt on medium speed. Beat well, scraping down sides, until completely smooth.

Line a baking sheet or a large platter with parchment paper. If possible, make room in your freezer. Use a teaspoon to create very small balls of filling, about 1/4 inch in diameter, or use a piping bag and 1/4 inch round piping tip to pipe out 1/4 inch blobs. Cover gently with plastic wrap. Put the sheet in the frezer and let cool for at least 20 minutes, until hardened, or up to overnight.

Heat oven to 350 F. Line a cookie sheet with parchment paper or a silicone baking mat. In a large mixing bowl or the bowl of a stand mixer using the paddle attachment, cream togeter the softened butter and sugar on medium speed until smooth and fluffy. Add egg and vanilla. Beat well, scraping down the sides and bottom of the bowl until smooth.

Remove the bowl from the stand and sift in the dry ingredients (flour, baking soda, salt, and cream of tartar). Return the bowl to the stand and beat the flour mixture into the butter mixture on low speed until just combined.

Pour the frozen cinnamon balls into the bowl into the bowl and stir until just combined.

In a small bowl, combine cinnamon and sugar for rolling.

Use a large cookie scoop to shape dough into balls. Roll dough in cinnamon-sugar mixture before transferring to the baking sheet. Leave at least 3 inches between cookie dough balls as these will spread.

Bake the balls of cookie dough 12 minutes. Do not let the cookies brown, or the baked cookies will be too tough. Bake until the tops of cookies are just set.

As soon as you remove the cookies from the oven, use a spoon to gently any errant cookie dough or cinnamon sugar mixture back into place so that each cookie forms nice circle. Let cool for 5 minutes on the baking sheet, then remove to a wire rack to fully cool.