Preheat oven to 350 degrees. Whisk the flour, baking powder and salt together in a bowl.
In a separate bowl, whip the butter and sugar together with a hand-held mixer (or strong arm) until fluffy, about 5 minutes. Beat in the egg and vanilla until combined. Mix in dry ingredients gradually.
Scoop the dough into 1-inch balls and roll in sugar. Place on a baking sheet, leaving space between them. Press a thumbprint into the center of each ball, about ½ inch deep. Fill in each indentation with about ¾ teaspoon jam or marmalade.
Bake cookies until the edges are golden, about 15 minutes. Cool cookies on baking sheets and serve!
Adapted from the Food Network.