Make the streusel. Whisk 0.5 cup flour, 0.5 cup brown sugar, and cinnamon. Pour in melted butter and toss with a fork until thick crumbs form. Chill in the fridge.
Mix the dough. Cream the softened butter, brown sugar, and granulated sugar for 2 minutes. Add the egg and vanilla. In a separate bowl, whisk 1.5 cups flour, cornstarch, baking soda, and salt. Gradually mix dry ingredients into wet until just combined.
Shape and fill. Preheat oven to 350°F (175°C). Scoop dough into 1.5-inch balls on lined baking sheets. Press a deep indentation into each ball. Pack a generous tablespoon of chilled streusel into each indentation.
Bake and cool. Bake for 11 to 13 minutes until edges are set. Let cool on the baking sheet for 5 minutes before moving to a wire rack to cool completely.
Glaze. Whisk powdered sugar and milk until smooth. Drizzle over the fully cooled cookies and let set before storing.