Whisk together the flour, baking powder, baking soda, corn starch and salt in a medium bowl.
2 1/4 all-purpose flour, 1/2 tbsp corn starch, 3/4 tsp baking powder, 1/4 tsp baking soda, 1/2 tsp salt
In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter and granulated sugar until light and fluffy, about 2–3 minutes. Add the egg and vanilla extract and mix until well combined. Gradually add the dry ingredients to the wet ingredients, mixing just until a soft dough forms. Be careful not to over mix the dough. Cover the dough and refrigerate for at least 1 hour.
1 cup butter, 1 cup sugar, 1 egg, 2 tsp vanilla extract
While the dough chills, combine the brown sugar, cinnamon, softened butter, flour and milk in a small bowl. Mix until you have a thick, cohesive filling. The filling should be soft enough to spread but firm enough to scoop and handle. If it feels too soft, refrigerate it for a few minutes.
1/2 cup brown sugar, 1/2 tbsp cinnamon, 1/2 tbsp flour, 2 tbsp butter, 1 tbsp milk
Preheat the oven to 350°F and line a baking sheet with parchment paper.
Using a one ounce cookie scoop (2 tablespoons), scoop a portion of cookie dough and flatten it into a small disc. Place a 1-2 teaspoons of the brown sugar cinnamon filling in the center, then cover with another 1/2 scoop of flattened dough. Pinch the edges together to completely seal the filling inside, then gently roll the cookie between your hands to smooth the edges. Flatten slightly and place on the prepared baking sheet, leaving space between each cookie. Repeat with the remaining dough and filling.
Bake the cookies for approximately 12-13 minutes, or until the edges are set and the centers still look slightly soft. Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.
Whisk together the powdered sugar, cinnamon, butter and milk until smooth. Start with a small amount of milk and add more as needed until you reach a thick but spreadable consistency.
1 cup powdered sugar, 1 tsp cinnamon, 2 tbsp butter, 1-2 tbsp milk
Once the cookies are completely cooled, drizzle or spread the cinnamon icing over the tops. Allow the icing to set before serving.