Step 1

In a bowl, mash together the melted butter, brown sugar, cinnamon, cornstarch and salt until completely combined. Measure out the mixture into ½ tablespoon pieces, roll into a ball, then add to a parchment-lined plate, and set aside.

Step 2

In another bowl, mix together the brown sugar, flour, rolled oats, cinnamon, salt, and melted butter with a fork until completely combined and crumbles form. Set aside.

Step 3

In the bowl of a stand mixer fitted with the paddle attachment (or with a hand mixer). Cream together the butter and sugars on medium-high speed for 5 minutes until light and fluffy. Scrape down the sides.

Step 4

Add the egg yolks and vanilla and beat on medium speed for another two minutes. Scrape down the sides again.

Step 5

Whisk together the flour, baking powder, baking soda, and salt and add to the creamed ingredients. Mix on low until just combined. Use a rubber spatula to scrape down the sides and mix in any leftover dry ingredients by hand.

Step 6

Preheat the oven to 350℉ and line two large cookie sheets with parchment paper.

Step 7

Using a 3 tablespoon cookie scoop, scoop the cookies and place six onto each cookie sheet. Flatten out each piece of cookie dough with your hands, then add a dollop of the cinnamon sugar filling. Tip: squish the cinnamon ball in between your fingers a few times to re-soften, then add to the cookie dough.

Step 8

Fold the dough around the filling and roll in between your hands to smooth. Repeat with all cookies.

Step 9

Press each cookie down gently with the palm of your hand until 1cm thick.

Step 10

Fluff the streusel topping with a fork, then press a heaping scoop of streusel onto the top of the cookie dough. Repeat with all cookies. Almost all streusel should be used.

Step 11

Bake one pan at a time for 8-10 minutes until the edges are set.

Step 12

Scoot the cookies immediately after they come out of the oven to sure up the sides, then cool completely on the pan.

Step 13

Whisk together the glaze, then drizzle over the fully cooled cookies.