Preheat the oven to 350°F (175°C) and line two large baking sheets with parchment paper.

In a medium mixing bowl, whisk together 2 1/4 cups flour, baking soda, salt, and 1 1/2 teaspoons cinnamon.

In a large bowl or stand mixer, cream together softened butter, 3/4 cup brown sugar, and granulated sugar for 2-3 minutes until light and fluffy.

Add the egg and 2 teaspoons vanilla extract, beating until fully incorporated.

Gradually incorporate the dry flour mixture into the wet ingredients at low speed until no streaks remain.

Prepare the streusel topping in a small bowl by combining 1/3 cup flour, 1/3 cup brown sugar, and 1/2 teaspoon cinnamon; use a fork or pastry cutter to cut in the cold butter until the mixture resembles coarse crumbs.

Scoop 1.5-tablespoon portions of dough and roll into balls, placing them 2 inches apart on the prepared sheets.

Use your thumb or a small spoon to create a slight indentation in the center of each dough ball and fill with a tablespoon of streusel, pressing it down gently.

Bake for 10-12 minutes until the edges are set and lightly golden.

Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack.

Whisk together powdered sugar, milk, and 1/4 teaspoon vanilla until smooth, then drizzle over the cooled cookies.