Preheat your oven to 350 degrees Fahrenheit and line two baking sheets with parchment paper.
In a small bowl, combine 6 tablespoons of all-purpose flour, 2 tablespoons of granulated sugar, 2 tablespoons of packed brown sugar, ½ teaspoon of ground cinnamon, and ⅛ teaspoon of salt. Whisk these ingredients together until they’re well combined.
Add 3 tablespoons of cold unsalted butter to the flour mixture. Using a pastry cutter or your fingers, cut the butter into the flour until it resembles coarse crumbs. Cover this mixture and refrigerate it until the cookie dough is ready.
In a large bowl, combine ½ cup of unsalted butter, ½ cup of packed brown sugar, and ⅓ cup of granulated sugar. Beat this mixture on medium-high speed with a hand mixer or a stand mixer fitted with a paddle attachment for about 3 to 4 minutes until light and fluffy.
Add in 1 large egg and ½ teaspoon of vanilla extract. Mix until just combined, scraping down the sides of the bowl as needed.
In a separate medium bowl, sift or whisk together 2 cups of all-purpose flour, 2 tablespoons of cornstarch, 1 teaspoon of baking powder, ½ teaspoon of baking soda, ½ teaspoon of ground cinnamon, and ½ teaspoon of salt. Gradually add this dry mixture to the wet mixture, mixing until just combined.
Using a medium cookie scoop (approximately 2.5 tablespoons), scoop the dough and place it onto the prepared baking sheets. Ensure to leave about 2 inches between each cookie.
With the back of the cookie scoop, gently press an indent into the tops of the cookies. Fill each indent with a generous amount of the streusel mixture you prepared earlier.
Bake the cookies in the preheated oven for about 11 to 13 minutes. Keep an eye on them; they won’t brown much, so look for the edges to be set.
Once baked, let the cookies cool on the baking sheet for 2 to 3 minutes before transferring them to a cooling rack to cool completely.
For the glaze, whisk together ¾ cup of powdered sugar, 2 tablespoons of milk, and optional ¼ teaspoon of vanilla extract until smooth.
Drizzle the glaze over the cooled cookies just before serving.