In a large bowl, cream 3/4 cup butter or margarine with the granulated sugar. Beat in the eggs, then the sour cream and vanilla. In another bowl, stir together the 3 cups flour, salt, and baking powder. Beat into the creamed mixture.
Turn half the batter into a greased and floured tube pan.
Sprinkle with half the streusel mixture.
Mix together 1/2 cup flour, 1/4 teaspoon baking powder, ground cinnamon, brown sugar, 2 tablespoons butter or margarine, and 1/4 cup chopped walnuts until crumbly.
Lay the apple slices on top and dot with the jelly. Cover with the remaining batter and sprinkle with the remaining streusel.
Bake the cake in a 350 degrees F (175 degrees C) oven for 1 hour, or until it tests done with a toothpick.