1. Mix Up the Streusel Grab a medium mixing bowl and whisk together the flour, light brown sugar, granulated sugar, cinnamon, and salt until evenly blended. Pour in the melted (but not hot) butter along with a small splash of vanilla. Stir everything together until the mixture turns clumpy and crumbly, similar to wet sand that holds together when pressed. Set the bowl aside — you’ll use this to fill the centers of your cookies in a few steps. If your kitchen runs warm, pop the bowl in the fridge while you make the dough so it stays crumbly rather than turning greasy.

2. Cream the Butter and Sugars In the bowl of a stand mixer fitted with the paddle attachment, add the softened butter, granulated sugar, and dark brown sugar. Beat on medium speed for 2 to 3 minutes, until the mixture turns noticeably lighter in color and looks fluffy and airy. Stop partway through to scrape down the sides and bottom of the bowl with a spatula, making sure nothing is hiding unmixed at the edges.

3. Add the Wet Ingredients With the mixer still running on low-to-medium speed, add the egg, egg yolk, milk, and vanilla extract. Continue mixing until the batter looks smooth, glossy, and fully combined, with no streaks of egg visible. Scrape down the bowl again if needed to make sure everything is evenly incorporated.

4. Whisk the Dry Ingredients In a separate bowl, whisk together the flour, baking powder, cinnamon, cornstarch, salt, and baking soda until evenly distributed. Whisking these together first ensures the leavening agents and spices don’t clump in one spot of your dough.

5. Bring the Dough Together With the mixer on low speed, add the dry ingredient mixture to the wet ingredients in two or three additions, mixing just until each addition disappears into the dough. Stop as soon as no dry streaks of flour remain — overmixing at this stage will make your cookies dense and tough instead of soft, so resist the urge to keep mixing “just to be safe.”

6. Scoop the Dough and Fill with Streusel Using a 2-tablespoon cookie scoop, portion the dough into balls — you should end up with roughly 18 to 20 cookies total. Roll each portion between your palms until smooth, then set them on a parchment-lined plate or small baking sheet (whatever fits in your fridge). Lightly dip the back of a tablespoon in flour, then press it into the center of each dough ball to create a deep indent. Fill each indent generously with about 2 to 3 teaspoons of the streusel mixture, pressing it in gently so it holds its place.

7. Chill the Dough Transfer the filled cookies, still on their plate or tray, into the refrigerator. Let them chill for 30 to 60 minutes. This resting time firms the butter back up, which keeps your cookies from spreading too thin in the oven and helps that streusel-filled center stay put instead of melting out to the edges. Make sure you actually have room in your fridge for the tray before you get to this step — a parchment-lined plate works well if space is tight.

8. Bake While the dough chills, preheat your oven to 350°F. Once preheated, arrange the chilled cookies a few inches apart on a parchment-lined baking sheet — baking 6 to 8 cookies per sheet at a time gives them enough room to spread properly. Bake for approximately 10 minutes, until the edges look set and lightly golden but the centers still appear a touch underbaked and soft. Resist the temptation to leave them in longer; they will continue cooking and firming up from residual heat once you pull them out.

9. Shape the Edges The instant the cookies come out of the oven, take a round glass or biscuit cutter slightly larger than the cookies themselves and gently rotate it around each one in a circular motion, “scooting” the edges into a perfectly round shape. Do this quickly, while the cookies are still hot and pliable. Let the cookies cool directly on the baking sheet for a few minutes to finish setting, then carefully transfer them to a wire rack to cool the rest of the way.

10. Make the Glaze and Finish Once the cookies have cooled completely, whisk together the powdered sugar, milk (or heavy cream), a pinch of salt, and vanilla extract in a small bowl until smooth and pourable. If the glaze looks too thick, add a touch more milk; if it looks too thin, whisk in a little more powdered sugar until you reach a consistency that drizzles slowly off a spoon. Drizzle the glaze generously over each cooled cookie, letting it pool into the streusel crevices. Let the glaze set for a few minutes before serving, and enjoy!