• Preheat the oven to 350 degrees Fahrenheit and line 2 baking sheets with parchment paper.

Streusel

• Add the flour, both sugars, cinnamon, and salt to a small bowl and whisk until combined.
• Add the butter and cut it in with a pastry cutter or fork until it is fully absorbed into the flour mixture. Alternatively, you may use clean hands to mash the butter into the flour mixture.
• Cover and refrigerate until the cookie dough is ready to be topped.

Cookies

• Add the butter and both sugars to a large bowl and beat on medium-high speed with a hand mixer or a stand mixer fitted with the paddle attachment for 3-4 minutes until light and fluffy.
• Add the egg and vanilla and mix until combined. Scrape down the sides and bottom of the bowl as needed.
• Sift or whisk together the flour, cornstarch, baking powder, baking soda, cinnamon, and salt in a medium bowl. Add the flour mixture to the butter mixture and mix until fully combined. The dough will be quite dry and crumbly.
• Use a medium cookie scoop (about 2 ½ tablespoons) to scoop the dough into cookies. You may have to press the dough into the scoop and level it off due to the crumbly texture.
• Place the cookie dough scoops onto the prepared baking sheets, leaving 2 inches between them. Use the back of the cookie scoop to gently press an indent into the tops of the cookies.
• Fill the indents with a generous amount of streusel.
• Bake for 11-13 minutes until the edges are set. The cookies won’t brown very much, if at all, so do not wait for them to start browning.
• Let the cookies cool on the baking sheet for 2-3 minutes before transferring to a cooling rack to finish cooling.

Glaze

• Once the cookies are completely cool, prepare the glaze by whisking together the powdered sugar, milk, and vanilla extract. Thicken the glaze by adding powdered sugar one teaspoon at a time, or thin it by adding ½ teaspoon of milk at a time until you reach a consistency you like.
• Drizzle the glaze across the cookies before serving.