Preheat oven to 350 degrees and lightly flour two cookie sheets. In a medium-size bowl, cream the butter and sugar until fluffy and light in color. Beat in the egg and the sour cream. Combine the flour, baking powder and salt and sift them into a bowl with the butter mixture. Knead the mixture by hand into a smooth dough .
Lightly flour a wooden board or counter. Carefully roll out the soft dough to an even thickness of slightly more than 1/4 inch. Using a round cookie cutter or glass about 2 1/2 inches in diameter dipped in flour and cut out the cookies. Gently transfer them to a cookie sheet with a spatula dipped in flour. Gather up the scraps, re-roll the dough and repeat. Prick the center
of each cookie in two places with a fork. Bake in the middle of the preheated oven until
lightly browned around the edges, 18 to 20 minutes. Remove the pan and transfer the cookies to wire racks to cool. Makes about three dozen cookies.