Preheat oven to 350°. In a large bowl, cream butter and granulated sugar until light and fluffy, 5-7 minutes. Beat in egg and vanilla. In another bowl, combine the flour, baking powder and salt; gradually add to creamed mixture and mix well., Divide dough into four portions; shape each into a 10x2-1/2-in. log. Place 4 in. apart on greased or foil-lined baking sheets. Make a 1/2-in. depression down the center of each log. Bake for 10 minutes., Fill depressions with jam. Bake until lightly browned, 10-15 minutes longer. Cool for 2 minutes. Remove to a cutting board; cut into 3/4-in. slices. Place on wire racks., In a small bowl, combine glaze ingredients. Drizzle over warm cookies. Cool completely.