Preheat your oven to 350°F (175°C). Grease and flour a 10-cup Bundt pan.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon. Set aside.

In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.

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

Gradually add the flour mixture to the butter mixture, alternating with the buttermilk, starting and ending with the flour. Mix until just combined.

In a small bowl, combine the brown sugar, ground cinnamon, and chopped nuts (if using) for the swirl.

Pour half of the cake batter into the prepared Bundt pan. Sprinkle the cinnamon swirl mixture evenly over the batter. Top with the remaining batter.

Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for 15 minutes before inverting onto a wire rack to cool completely.