Preheat oven to 350 F
Make the streusel by mixing together the flour, brown sugar, cinnamon and salt
Add the cubed butter and use your fingers to work into the dry ingredients to form clumps similar to wet sand texture
Chill streusel in fridge until ready to use
Make the dough by stirring together the softened butter and sugars until combined
Mix in the egg yolk and vanilla extract until well combined
Add the flour, cornstarch, cinnamon, baking soda/powder, and salt to the dough and mix until combined
Pop the dough in the fridge for 5-10 minutes while oven finishes preheating
Scoop dough onto a parchment lined baking sheet into two large balls
Use your thumbs to make large indent in each ball
Spoon in the streusel filling
Bake for 7-10 minutes until the edges are set and just golden brown
Allow to cool
Optional: Microwave the butter, brown sugar, and heavy cream for 20-30 seconds until melted and bubbly
Mix in the vanilla and powdered sugar, adding more cream if needed to thin to desired thickness
Pipe icing over the cookies and enjoy
