Powdered sugar; for dusting Mix together the filling, and put aside. Preheat oven to 350 degrees. For the cake, sift together the flours, baking powder, soda and salt in a large bowl. In another bowl, cream the butter, sugar and sour cream until light and fluffy. Add the eggs, one at a time, beating well after each. Stir in the lemon zest. Add the flour mixture, and beat until just smooth. Pour about half the batter into a greased and floured 25 cm bundt or tube pan. Sprinkle the filling evenly down the center of the batter...preventing the filling from touching the sides of the pan. Cover with the remaining batter. Bake until a skewer tests clean in the thickest part, about 45-55 minutes. Cool in the pan for 10 minutes, before turning out. Dust with powdered sugar. ----