Preheat oven to 350°. Grease and flour a 10-in. tube pan. In a large bowl, toss apples with bourbon., In a large bowl, beat sugar, oil, eggs and vanilla until well blended. In another bowl, whisk flour, pie spice, salt and baking soda; gradually beat into sugar mixture. Stir in apple mixture and walnuts., Transfer to prepared pan. Bake until a toothpick inserted in center comes out clean, 1-1/4 to 1-1/2 hours. Cool in pan 10 minutes before removing to a wire rack., In a small heavy saucepan, combine butter, brown sugar, milk, bourbon and salt. Bring to a boil over medium heat, stirring occasionally; cook and stir 3 minutes. Remove from heat; stir in vanilla. Drizzle over warm cake. Cool completely.