With an electric mixer, beat ½ cup softened butter, and granulated sugar until creamy. Stir in egg yolk and vanilla extract and beat until combined
Whisk the flour and cornstarch in a separate bowl. Gradually beat dry ingredients into the wet mixture until combined.
Roll dough into a disk, wrap it in plastic film and chill for 15 minutes.
To make the filling, mix brown sugar, cinnamon, and ⅛ teaspoon salt.
Roll chilled dough into a 12x8-inch rectangle.
Spread the softened butter, leaving a ½-inch border. Sprinkle with the sugar-cinnamon mixture.
Preheat oven to 375°F (190°C).
Roll dough tightly into a log, pinch the seams, wrap it in plastic wrap, and freeze for 15 minutes.
Slice dough into ¼-inch thick slices and bake for 10-12 minutes until the edges are lightly golden. Cool on the baking sheet for a few minutes and transfer to a cooling rack to cool down completely.
Whisk the powdered sugar, 2 teaspoons of milk, and vanilla extract to make a glaze. If the glaze is too thick, add another teaspoon of milk.
Drizzle over cooled cookies and let harden before serving.
Notes
Use softened butter. This helps prevent the cookies from spreading along with keeping the dough from crumbling. In the filling, it keeps the spread smooth.Chill the dough. Important step to properly ensure the dough stays together and the cookies don’t spread. Don’t skip this step.Do not overdo the filling. Spread the filling evenly leaving a ½ inch border around the dough so that it does not overflow. If too much is used the cookies will be messy.Freeze before slicing. By freezing the dough for at least 15 minutes, the log of dough will keep its shape making it easier to slice.Use a sharp knife to slice. The slices should be ¼ inch thick using a sharp knife to make a clean cut.Check for doneness. Every oven is different so keep watch. The cookies should be done when the edges start to brown. The centers will be soft.Let cool before serving. Transfer the cookies to a cooling rack. Once cooled, apply the glaze and let harden before enjoying.