Preheat the oven to 350°F (175°C).

Grease the bundt pan thoroughly with butter or cooking spray.

In a mixing bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and sugar.

In another bowl, cream the softened butter and granulated sugar until light and fluffy.

Beat in the eggs one at a time, followed by the vanilla extract.

Gradually add the dry mixture into the butter mixture, alternating with the sour cream.

Gently fold in the blueberries and walnuts, if using.

Pour the batter into the prepared bundt pan and smooth the top.

Bake for 50 minutes or until a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for about 10 minutes before inverting it onto a wire rack.