Preheat your oven to 375°F (190°C) and line two baking sheets with parchment paper.

In a stand mixer, combine the unsalted butter, brown sugar, and granulated sugar. Beat on medium-high for 3-4 minutes until light and fluffy.

Add the egg, egg yolk, and vanilla extract to the butter mixture. Lower the mixer speed and combine until fully incorporated.

Gradually mix in the baking powder, baking soda, salt, cornstarch, cinnamon, and all-purpose flour until just combined.

Scoop the dough onto the prepared baking sheets, spacing them about 2 inches apart and create an indentation in the center.

In a separate bowl, mix the flour, brown sugar, cinnamon, and a pinch of salt. Add softened butter and blend until it resembles coarse crumbs.

Distribute the crumb topping evenly over the cookies' indentation, pressing gently.

Bake for 9-12 minutes, or until edges are golden brown.

Let cookies cool completely on the baking sheets. Prepare the glaze by mixing powdered sugar, a sprinkle of cinnamon, and milk until desired consistency.

Drizzle the glaze over the cooled cookies.