Preheat oven to 375 degrees F and cover a baking sheet with parchment paper.
In a large bowl, cream together the butter and sugar. Once combined, mix in the egg and vanilla.
In a separate bowl, combine the flour, cornstarch, baking powder, and salt. Add the dry ingredients to the creamed mixture and blend thoroughly. The dough will be soft. If it is too sticky to shape into balls, refrigerate dough for 20-30 minutes until it is easy to handle.
Shape the dough into 1-inch balls. Place them 2 inches apart on the baking sheet. Using your thumb or finger, press an indentation, about halfway down, into the center of each cookie.
Fill the center of the cookies with a ¼ teaspoon of jam. Do not overfill.
Bake 8-10 minutes or until the cookies are light brown on bottom and pale on top.
Remove the cookies from the oven and place on a cooling rack. Using a sifter or small sieve, lightly dust cookies with powdered sugar. If necessary, fill cookies with more jam.
Recipe From: Brio Magazine February 2019 Berry Thumbprint Cookies