Place the flour, baking powder, cinnamon, and salt in a large mixing bowl and whisk until combined and set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, granulated sugar, and brown sugar for 3-5 minutes until light and creamy. Add the egg and vanilla and beat on medium until no more egg is visible. Start on low speed (3-4 on a stand mixer) then gradually increase the speed to high (7-9).

Add the flour mixture to the bowl with the butter and beat until no more streaks of dry flour are visible. Start at low speed and gradually increase to high speed. Be sure to scrape down the bottom and sides of the bowl to make sure everything gets evenly mixed.

Lay plastic wrap on a large flat surface and scoop the dough out onto the plastic wrap. Pat the dough into a rectangle, wrap the dough in plastic wrap, and refrigerate it for one hour.

Right before taking the dough out of the refrigerator, add the melted butter, brown sugar, and cinnamon to a small bowl and mix with a fork until combined and it resembles wet sand.

After one hour, take the dough out of the refrigerator, remove the plastic wrap, and place it on a lightly floured surface. Lightly dust a rolling pin with flour and roll the dough out into a ¼-inch thick rectangle.

Spoon the cinnamon, sugar, and butter mixture all over the cookie dough, getting as close to the edges as possible. Use your fingers to evenly distribute the cinnamon sugar on top of the cookie dough, but do not firmly pat or press it into the dough.

Starting with one of the long edges, carefully lift the dough, tightly roll it into a log, and seal the seam by patting the cookie dough together to fully enclose the cinnamon-sugar layer. If you notice the cookie dough cracking in places, just use your fingers to seal it back together.

Remove the plastic wrap from the cookie dough log and slice it into ½-inch thick cookies. Place the cookies on the baking sheet about 2 inches apart and bake them in the preheated oven for 8-10 minutes, or until the edges just start to turn light golden brown.

Allow the cookies to cool for 5 minutes on the baking sheet then transfer them to a wire rack to cool completely. While the cookies are cooling, whisk together the powdered sugar, milk, and vanilla until smooth, and use a spoon to drizzle the glaze over the completely cooled cookies.