Making the Streusel Filling:

In a medium bowl, mix together the flour, brown sugar, cinnamon, and salt.

Add the softened butter and use your fingers to blend until it forms small clumps.

Refrigerate the mixture.

Preparing the Cookies:

Preheat your oven to 400°F and line two baking sheets with parchment paper or silicone mats.

Mixing Dry Ingredients:

In a large bowl, combine the all-purpose flour, cake flour, cornstarch, cinnamon, baking soda, baking powder, and salt.

Creaming Butter and Sugars:

Put the cubed butter and sugars into a stand mixer with the paddle attachment.

Mix on medium speed until smooth, about 1-2 minutes.

Adding Wet Ingredients:

Add the egg, egg yolk, and vanilla extract to the butter and sugar mixture.

Use a spatula to scrape down the sides of the bowl if needed, then mix again.

Combining Wet and Dry Ingredients:

Gradually add the dry ingredients to the wet mixture, about 1/4 cup at a time. Mix until combined.

Shaping the Cookies:

Scoop and roll the dough into about 8 large balls (or smaller if you prefer).

Place them on the prepared baking sheets.

Make an indent in each cookie with your thumb or the end of a utensil.

Filling and Baking:

Spoon about 1 Tbsp. of the streusel filling into each indent.

Bake for about 9-11 minutes, until the edges are set and slightly golden. They’ll continue to bake on the sheet after you take them out, so pull them out just before they’re completely done to avoid over-baking.

Optional Step for Perfectly Round Cookies:

Right after the cookies come out of the oven, place a circular cutter or glass around each cookie and give them a spin to smooth out the edges.

Finishing Touches:

If you have any leftover streusel, add more to the centers of the cookies.

Let the cookies rest on the baking sheet for about 5 minutes, then transfer them to a cooling rack.

Making the Icing:

Whisk together the melted butter, heavy cream, powdered sugar, vanilla, and a pinch of salt.

Once the cookies are cool, drizzle the icing over the top and enjoy!