Set the oven to 375°F and cover 2 baking sheets with parchment paper. The lined surfaces will help release the cookies without disturbing their crumb topping.
Fit a stand mixer with its paddle and cream the dough butter with both sugars at medium-high speed for 3 to 4 minutes. Look for a paler mixture with a fluffy texture rather than a dense, buttery paste.
Switch to low speed and incorporate the whole egg, extra yolk, and vanilla. Continue until the mixture looks smooth and evenly combined.
With the mixer running slowly, gradually incorporate the baking powder, baking soda, salt, cornstarch, and flour, stopping before every trace of flour disappears. A few flour streaks around the bowl signal that it is time to stop rather than keep beating the dough.
Portion the dough into 3-tablespoon balls, arrange 6 on each sheet, and press a deep hollow into each center with the scoop’s back or your fingers. Keep a raised rim around each indentation to cradle the streusel.
Combine the topping flour, brown sugar, salt, and cinnamon in a small bowl, work in the soft butter with a fork or your fingers, and fill each dough hollow with a handful of crumbs. The topping should gather into clumps that separate easily rather than smear into a paste.
Bake for 9 to 12 minutes, guide a spoon or spatula around the hot cookie edges to round them, and leave the cookies to cool completely before glazing. The edges should just begin turning golden when the cookies come out of the oven.
Stir the powdered sugar, cinnamon, and milk into a glaze, then drizzle it over the completely cooled cookies. The glaze should leave distinct ribbons across the topping instead of disappearing into it.