For the Streusel: Mix together flour, baking powder, cinnamon, dark brown soft sugar, butter and walnuts until crumbly. Set aside.
For the Cake: Cream together butter and sugar. Beat in the eggs, then soured cream and vanilla. In another bowl, stir together the flour, salt and baking powder. Beat into the creamed mixture.
Pour half of the batter into a greased and floured tube cake tin. Sprinkle with half the streusel mixture. Arrange apples on top and dot with jam. Pour over remaining batter and sprinkle with remaining streusel.
Bake the cake at 180 C / Gas 4 oven for 1 hour or until it tests done with a skewer. Transfer to a rack to cool.