Preheat your oven to 350°F. Line two baking sheets with parchment paper.
Make the streusel: In a medium bowl, combine 6 tbsp softened unsalted butter, 3/4 cup packed brown sugar, 3/4 cup spooned-and-leveled all purpose flour, 2 1/2 tsp pumpkin pie spice, and a pinch of salt. Use a fork or your fingers to work everything together until it forms clumps ranging from pea-sized to marble-sized. Don't over-mix it into a paste — you want uneven pieces so some are crunchy and some are crumbly. Set it aside.
Mix the dry ingredients: In another medium bowl, whisk together 1 3/4 cups spooned-and-leveled all purpose flour, 1 tbsp pumpkin pie spice, 1/2 tsp baking powder, 1/2 tsp baking soda, and 1/2 tsp salt. Set this aside too.
Cream the butter and sugar: In a large bowl, beat 3/4 cup softened unsalted butter and 1 cup packed brown sugar together on medium speed until light and fluffy — about 3 minutes. Scrape the sides of the bowl halfway through. You'll know it's ready when the mixture looks pale and feels airy when you lift the beater.
Add the wet ingredients: Beat in 2 room-temperature egg yolks, one at a time, then 2 tsp vanilla extract. Scrape the bowl again. The mixture should look smooth and glossy. If it looks curdled, your butter was too cold or your egg yolks were too cold — but don't panic, it'll come together once you add the dry ingredients.
Add the pumpkin: Beat in 1/2 cup pumpkin puree on low speed until just combined. The dough will look a little wet at this point — that's normal.
Add the dry ingredients: Gradually add the flour mixture to the wet mixture, mixing on low speed until just combined. Stop as soon as no streaks of flour remain. Over-mixing at this stage develops gluten and makes the cookies tough instead of tender.
Scoop and top: Scoop the dough into 1.5-tablespoon portions (about the size of a golf ball) and place them 2 inches apart on the prepared baking sheets. Gently press a generous pinch of streusel onto the top of each dough ball — don't press it into the dough, just let it sit on top. I use about 1 tablespoon of streusel per cookie.
Chill the scooped dough: Pop the baking sheets into the fridge for 20 minutes. This is the step I begged you not to skip. Chilling firms up the butter so the cookies spread less and the streusel stays on top instead of sinking into the dough.
Bake: Bake at 350°F for 10-12 minutes. At 10 minutes, the edges should be set and the centres should look slightly puffed and just barely golden. If you touch the centre lightly, it should feel soft but not raw. Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Make the glaze: While the cookies cool, whisk together 1 cup powdered sugar, 1 tsp vanilla extract, and 2 tsp whole milk. If it's too thick to drizzle, add another teaspoon of milk. If it's too thin, add a tablespoon more powdered sugar. The consistency should be like thick cream — it should flow off the whisk in a ribbon but hold its shape for a second before disappearing.
Glaze the cookies: Once the cookies are completely cool, drizzle the glaze over the tops using a spoon or a piping bag with a tiny tip cut off. I like to zigzag it over the streusel so every bite gets a little sweetness. Let the glaze set for about 15 minutes before stacking or serving.