Beat butter for 10 minutes with electric mixer until very light and creamy. Beat the egg yolks and vanilla in a separate bowl until lemon-colored, 1 minute. Add 1/2 cup sugar and beat for 10 minutes. Add egg mixture to butter and add brandy and the flour, a little bit at a time, until dough is soft.
Shape tablespoonfuls of dough into small crescents or balls and place on an ungreased baking sheet. Bake until very lightly browned, about 25 minutes. Cool for about 10 minutes on the baking sheets. Sprinkle liberally with sifted confectioners' sugar while the cookies are still warm. Continue to cool to room temperature.
Cook's Note: If the dough gets too warm and sticky while rolling the balls, re-chill it before continuing.