In a small bowl, combine 4 tablespoons of softened unsalted butter, ⅓ cup of packed brown sugar, 1½ tablespoons of all-purpose flour, 2½ teaspoons of ground cinnamon, and a pinch of salt. Mix until you achieve a smooth paste. Set aside.

Beat together 1 cup of unsalted butter and 1⅓ cups of granulated sugar in a large mixing bowl until creamy—around 2-3 minutes.

Add in 1¾ teaspoons of pure vanilla extract, 1 large egg, and 1 large egg yolk; continue mixing until well combined.

In another bowl, whisk together 2¾ cups of all-purpose flour, ¾ teaspoon of baking powder, 1 teaspoon of cornstarch, and ¼ teaspoon salt. Gradually blend this dry mixture into the wet mixture until just combined.

Cover the dough with plastic wrap and chill for at least 30 minutes.

Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.

Scoop out a tablespoon of dough, flatten it slightly, and place a dollop of your cinnamon filling in the center. Fold the edges of the dough over the filling and roll it into a ball. Repeat until all the dough is used.

Place the cookies on the prepared baking sheets and bake for about 10-12 minutes or until the edges are just slightly golden.

Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.