Step 1Heat the oven to 350 degrees. Butter and flour a 10-inch tube pan.
Step 2Mix together the all-purpose and cake flours, the baking soda and baking powder in a large bowl.
Step 3In a separate bowl, beat the butter with the sugar until fluffy and light. Add the egg and vanilla and mix well. Add half the flour mixture, mixing just until the flour is blended. Blend in the sour cream, then the remaining flour mixture.
Step 4Spread half the batter carefully in the pan. Sprinkle with half the Topping and spread with the remaining batter. Sprinkle the top of the cake with the remaining topping.
Step 5Bake until a toothpick inserted in the middle comes out clean, 40 to 45 minutes.
Step 1Mix together the flour, sugar, salt and walnuts. Add the butter in small pieces. Rub the butter in by hand until the mixture is crumbly. Be careful not to over-mix.