To Make the Dough:

Using a hand-held or a standing mixer, cream the butter and sugars together until fluffy.  Add the egg and vanilla, beating well to incorporate.  Stir in all-purpose flour, cornstarch, baking powder and soda, and salt.  Stir. until. mixture comes together.  

Use a #50 scoop (3 T.) to form dough balls.  Place on a parchment or Silpat lined baking sheet.  Set aside.*

To Make Filling:

In a medium-sized bowl, combine brown sugar, butter, flour and cinnamon.  Use a spatula or the back of a spoon to press the mixture together until it becomes a uniformly mixed paste.  

To Assemble Cookies:

Flatten each dough ball into a 3-inch wide disk.  Spoon about 1 tsp. of filling into the center of each disk.  Press the dough up and around the filling. Roll dough into a smooth ball and return to baking sheet.  Repeat with remaining dough and filling.  Refrigerate dough balls for about 1 hour.

Towards the end of the dough’s chill time, preheat oven to 350 degrees F.  Space dough balls out about 2-inches on the baking sheet.  Bake for 11-14 minutes, or until the dough tops appears set and the edges appear golden brown.  Cool cookies on a wire rack.  

To Make Icing:

Combine all icing ingredients in a small bowl, whisking until smooth.  Spread about 1 teaspoon of icing over the top of each cookie.  Icing will set.  Cookies can be stored at room temperature in a tightly sealed container for up to 3 days.