3) In another bowl, whip the butter and the sugar with a hand-held mixer until fluffy, about five minutes. Beat in the egg and vanilla until just combined. Slowly beat in the dry ingredients in two additions, mixing just until incorporated.
4) Scoop the dough into 2-cm balls with an ice cream scoop and roll in sugar. Place about 5-cm apart on the prepared baking sheets. Press a thumbprint into the center of each ball, about 1-cm deep. Fill each indentation with about three-quarters teaspoon of jam.
5) Bake cookies until the edges are golden, about 15 minutes. (For even colour, rotate the pans from top to bottom about halfway through baking.) Cool cookies on the baking sheets. Serve.
6) Store cookies in a tightly sealed container for up to five days.