Preheat your oven to 400°F (205°C) and generously grease a 10-inch Bundt pan to prevent sticking.

In a large mixing bowl, use an electric mixer to cream together the sugar and butter until the mixture is light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking soda, and baking powder. Add the dry ingredients to the wet mixture in two batches, alternating with the sour cream. Beat briefly after each addition until just combined. Gently fold in the chopped walnuts.

Pour half of the batter into the prepared Bundt pan and spread it evenly.

In a small bowl, mix the sugar and cinnamon for the filling. Sprinkle this mixture evenly over the batter in the pan.

Spoon the remaining batter on top of the filling in heaping spoonfuls, doing your best to cover the cinnamon layer completely.

Place the pan in the preheated oven and bake for 8 minutes. Then reduce the oven temperature to 350°F (175°C) and continue baking for about 40 more minutes, or until a toothpick inserted in the center of the cake comes out clean.

Allow the cake to cool in the pan on a wire rack for 5 minutes. Carefully invert the cake onto a serving plate or wire rack and let it cool completely before serving.