Preheat oven to 350°F (175°C). Grease and flour a 9-inch bundt or tube pan.
Cream the butter and 1 cup sugar until light and fluffy. Add eggs one at a time, beating well after each.
Stir in sour cream and vanilla. In a separate bowl, whisk together flour, baking powder, and salt.
Gradually mix the dry ingredients into the wet mixture until just combined.
Fold in chopped apples and walnuts, if using.
Pour half the batter into the prepared pan. Sprinkle half the cinnamon-sugar mixture on top. Add the remaining batter, then sprinkle the rest of the cinnamon-sugar.
Bake for 55–60 minutes or until a toothpick inserted in the center comes out clean.
Cool in the pan for 15 minutes, then turn out onto a rack to cool completely.