1. In a medium bowl, beat butter and superfine sugar with an electric mixer on medium speed until light and fluffy. Beat in egg, cognac, and vanilla.
2. Add flour mixed with baking powder to sugar mixture and beat until blended. Cover and refrigerate dough at least 1 hour, until firm.
3. Preheat oven to 350°F. Shape dough into 1-inch balls and place 1 inch apart on ungreased cookie sheets.
4. Bake 10 to 12 minutes, or until edges just begin to turn golden brown. Remove to a rack to cool slightly, about 10 minutes. Roll in powdered sugar. Let cool completely before serving.