Preheat oven to 350°. In a large bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine flour, baking powder and salt; add to creamed mixture just until combined.
Spread half of the batter into a greased 15x10x1-in. baking pan. Spread with pie filling; sprinkle with nutmeg. Drop remaining batter by teaspoonfuls over the top.
Bake until golden brown, 40-45 minutes. In a small bowl, combine glaze ingredients until smooth. Drizzle over warm bars. Cool completely on a wire rack.