Begin by melting the butter and allowing it to cool to almost room temperature. In a large mixing bowl, combine the melted butter with the brown sugar, stirring until the mixture is smooth and well-blended.

Next, add the egg, egg yolk, and vanilla extract to the butter and sugar mixture. Stir thoroughly until everything is well-incorporated.

In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, baking powder, and salt. Ensure the dry ingredients are evenly mixed.

Gradually introduce the dry mixture into the wet ingredients, stirring until everything is combined into a smooth dough. Cover the bowl with plastic wrap and refrigerate for at least 45 minutes to allow the dough to firm up.

When ready to bake, preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or use an ungreased cookie sheet if parchment paper is unavailable.

Remove the chilled dough from the refrigerator. Scoop out dough portions roughly the size of 2 tablespoons. Gently roll each portion into a ball, then roll it in granulated sugar until fully coated.

Place the sugar-coated dough balls on the prepared baking sheet, ensuring they are spaced at least 2 inches apart to allow for spreading.

Bake the cookies in the preheated oven for 11-12 minutes. The centers may appear slightly underdone; this is normal. Allow the cookies to cool completely on the baking sheet. They will finish cooking and firm up as they cool, making them too delicate to move while still hot.