Preheat oven to 325 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 the sugar with a hand-held 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.
Roll dough into 1-inch balls and roll in sugar. Place about 2-inches apart on the prepared bakingsheets. Press a thumbprint into the center of each ball, about 1/2-inch deep. Fill each indentation with about 3/4 teaspoon jam.
Bake cookies until the edges are golden, about 12 minutes. (For even color, rotate the pans from top to bottom about halfway through baking.) Cool cookies on the baking sheets.
Store cookies in a tightly sealed container for up to 5 days.
