Preheat the oven to 350°F. Line a large baking sheet with parchment paper.

In a large mixing bowl, use a handheld mixer on medium-high speed to cream together the butter, ½ cup granulated sugar, and brown sugar until light and fluffy. Add the applesauce, vanilla, cream of tartar, baking soda, and salt. Beat on medium until creamy and combined.

Add the flour. Beat on low until a soft dough forms. You may need to go in with a wooden spoon or rubber spatula to press the dough together.

To make the cinnamon sugar, whisk together ¼ cup granulated sugar and 1 tbsp cinnamon. Use a 1.5 tbsp cookie scoop to portion dough. Roll the dough into a bowl, coat in cinnamon sugar mixture, then roll again to form an even ball. Place dough balls on prepared baking sheet.

If making the streusel: In a medium-sized mixing bowl, whisk together the flour, sugar, cinnamon, and salt. Add the softened butter and use a fork to mash into the flour until fully incorporated. Mixture should feel like wet sand. Make a small indent into the center of each dough ball using your thumb. Spoon in a heaping tbsp of streusel. Slightly press streusel down into the dough to secure.

Bake cookies for 11 minutes. Cool for 10 minutes on baking sheet before transferring to a wire cooling rack. Drizzle with icing, if desired. Enjoy!