In a small bowl, combine flour, baking powder and salt. Set aside.
In a large bowl, whisk together egg whites, vanilla, butter and milk. Add peanut butter and stir until all of the liquid is combined with the peanut butter. Stir in the brown sugar.
Using a spoon at first and then mixing with your hands, add the flour mixture. It will be hard to incorporate all of it using just a spoon, so get in there and combine it all with your hands.
Roll the dough into one inch balls and place on baking sheet. Use your thumb or index finger to indent the middle of each cookie. Fill each indentation with fruit spread.
Bake 10-12 minutes. Allow the cookies to cool for two minutes on the baking sheet before removing.
Recipe by Down Home Inspiration at https://www.downhomeinspiration.com/peanut-butter-and-jelly-thumbprint-cookies/