Preheat your oven to 375°F (190°C). Line two large baking sheets with parchment paper. In a medium bowl, whisk together the flour, baking soda, salt, and ground cinnamon. Set aside.

In a large bowl, using an electric mixer, cream together the softened unsalted butter, granulated sugar, and light brown sugar on medium speed until light and fluffy, about 2-3 minutes. Beat in the large egg and vanilla extract until well combined, scraping down the sides of the bowl as needed.

Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix! Gently fold in the finely diced apple until evenly distributed throughout the dough.

Drop rounded tablespoons of dough (or use a 1.5-inch cookie scoop) onto the prepared baking sheets, leaving about 2 inches between cookies. Bake for 10-12 minutes, or until the edges are lightly golden and the centers are just set. They might look slightly underdone, but they'll firm up as they cool.

Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely. This helps them set and prevents breaking.

While the cookies cool, prepare the glaze. In a small bowl, whisk together the powdered sugar and 2 tablespoons of milk until smooth. If the glaze is too thick, add more milk, 1 teaspoon at a time, until it reaches a drizzling consistency. Drizzle the glaze over the completely cooled cookies before serving.