Preheat oven to 350 degrees F. Line 2 baking sheets with parchment paper or silicone mats.
Whisk the flour, baking powder and salt together in a bowl.
In another bowl, whip the butter and granulated sugar with a mixer until fluffy, about 5 minutes.
Beat in the egg and vanilla until just combined. Slowly beat in the dry ingredients in 2 additions, mixing just until incorporated.
Scoop the dough into 1-inch balls with a cookie or ice cream scoop and roll in fine sugar. Place about 2-inches apart on the prepared baking sheets. Press a thumbprint into the center of each ball, and ½-inch deep. Fill each indentation with about ¾ teaspoon jam.
Bake cookies until the edges are barely brown, about 10 – 15 minutes. (For even color, rotate the pans from top to bottom about halfway through baking). Cool cookies on baking sheets. Serve.
Store cookies in a tightly sealed container for up to 5 days.
This recipe was provided by Irene M