Instructions

1. Arrange 2 racks to divide the oven into thirds, and heat the oven to 350°F. Line 2 baking sheets with parchment paper.
2. Beat 1 stick room-temperature unsalted butter, 1/2 cup granulated sugar, 1/2 cup packed light brown sugar, and 1/2 teaspoon kosher salt with the paddle attachment in a stand mixer on medium-high speed until light and fluffy, about 2 minutes.
3. Scrape down the sides and bottom of the bowl with a flexible spatula. Beat in 1 room-temperature large egg and 2 teaspoons vanilla extract on medium-high speed until smooth and fluffy, about 1 minute. Beat in 1 3/4 cups all-purpose flour, 1/2 teaspoon baking powder, and 1/4 teaspoon baking soda on low speed until combined and no streaks of flour remain; the dough should form a cohesive mass.
4. Stir 1/4 cup packed light brown sugar and 2 teaspoons ground cinnamon together in a small bowl with a fork. Sprinkle over the cookie dough, then use the flexible spatula to fold it into the dough a few times. Do not overmix — you’ll lose your swirls!
5. Stir 1/2 cup all-purpose flour, 3 tablespoons packed light brown sugar, and 1 pinch kosher salt together in the now-empty cinnamon swirl bowl. Stir in 3 tablespoons melted unsalted butter with the fork or your fingers until it resembles wet sand and clumps together.
6. Scoop out the dough in 3-tablespoon (1 1/2-ounce) portions (about 14). Roll each portion in your hands to smooth the edges, then place at least 2 inches apart on the baking sheets, no more than 8 per sheet.
7. Flatten the top of the balls gently with your palm until about 2 inches wide and 3/4-inch thick. Top each cookie with about 1 teaspoon of the streusel topping (you will not use up all the streusel at this point), then gently press the streusel into the top and sides of the dough so it sticks.
8. Bake the cookies for 7 minutes. Take the baking sheets out of the oven. Sprinkle the remaining streusel evenly over the cookies and gently press it in. Return the cookies to the oven, switching the sheets between racks, and bake until they’ve puffed up and are set and firm around the edges but still soft in the middle, 5 to 8 minutes more.
9. Let the cookies cool on the baking sheets for 5 minutes. Use a flat spatula to transfer the cookies to a wire rack and let cool completely, about 20 minutes. Reserve one of the parchment sheets.
10. Return the cookies to the reserved parchment.
11. Whisk 1 cup powdered sugar, 2 tablespoons whole milk, 1/2 teaspoon vanilla extract, and 1 pinch kosher salt together in a small microwave-safe bowl until smooth. The mixture will be thick.
12. Microwave for 15 seconds, then stir again — this will thin the icing out and help it harden once it dries on top of the cookies. Use a spoon to drizzle the cookies with the icing. Let sit until the icing sets, about 15 minutes.