Preheat the oven to 350°F (175°C). Grease and flour a bundt cake pan.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In another bowl, whisk together the flour, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, alternating with the sour cream, until just combined.

In a small bowl, mix together the brown sugar, ground cinnamon, and chopped walnuts (if using).

Pour half of the batter into the prepared bundt pan. Sprinkle half of the cinnamon swirl mixture over the batter.

Pour the remaining batter on top and then sprinkle the rest of the cinnamon swirl mixture.

Use a knife to gently swirl the batter and cinnamon mixture together.

Bake in the preheated oven for 45-50 minutes or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for 10 minutes before inverting onto a cooling rack.

In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth.

Drizzle the glaze over the cooled bundt cake before serving.