Brown butter from the cookie batter in a saucepan over medium heat until golden and nutty.

Remove from heat, pour in a large mixing bowl and stir in milk powder. Refrigerate until solid but scoopable, about 45–60 minutes. Do not skip this step. If the butter is warm or melted when mixing, the cookies will spread too much.

To make the streusel, in the same saucepan, brown 6 TBSP butter until lightly amber and fragrant. Remove from heat. In a separate mixing bowl, combine the rest of the ingredients from the brown butter cinnamon streusel list: brown sugar, granulated sugar, flour, cinnamon powder, vanilla extract and salt. Pour the browned butter over the dry ingredients and mix until crumbly. Use your hands to create a mix of small and larger crumbs for texture. Refrigerate the streusel mixture.

When the brown butter has solidified, cream it with the granulated sugar and light brown sugar until light and fluffy. A stand mixer or handheld electric mixer works best. Add the egg, vanilla extract, and sour cream. Mix until smooth and fully combined.

In a separate bowl, whisk together all the dry ingredients: flour, cornstarch, baking powder, baking soda, salt and cinnamon.

Add the dry ingredients to the wet ingredients and mix with a silicone spatula until just combined. Do not overmix. Fold in one third of the chilled streusel mixture. Approximately 1 cup (120g). Cover and refrigerate the dough for at least 1 hour.

Preheat oven to 350°F. Line a baking sheet with parchment paper. Divide the dough into 10 large portions. Roll into balls and place spaced at least 3 inches apart on the prepared baking sheet. Using the back of a spoon, gently press the center of each dough ball to create a shallow well. Fill each indentation generously with the remaining streusel topping. For extra thick bakery-style cookies, chill the shaped dough for an additional 15–20 minutes before baking. This is optional.

Bake at 350°F for 11–13 minutes, until the edges are set and lightly golden. The centers should look slightly soft. If the cookies spread more than desired, immediately place a large round object like a mug or bowl around the cookie and gently swirl to reshape into a perfect circle.

Cool the baked cookies for 5 minutes on the sheet pan, then transfer baked cookies to a cooling rack to fully cool. The centers will appear underbaked at first and will continue to cook through even on the cooling rack.

Cool completely before dusting with powdered sugar or drizzling with glaze.