In a mixing bowl, combine oil and sugars. Add eggs, one at a time, beating well after each addition. Combine dry ingredients; add to batter and stir well. Fold in apples, walnuts and vanilla. Pour into a greased and floured 10-inch tube pan. Bake at 325 degrees for 1 1⁄2 hours or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack to cool completely.
In the top of a double boiler over simmering water, heat brown sugar, cream, butter and salt until sugar is dissolved. Cool to room temperature. Beat in confectioners' sugar until smooth; drizzle over cake. Sprinkle with nuts if desired.