Preheat your oven to 350°F (175°C). Grease a 10-cup Bundt pan generously with butter or cooking spray.

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

In a large mixing bowl, use an electric mixer to cream the softened butter and granulated sugar together on medium speed for 3-4 minutes.

Add the eggs, one at a time, mixing well after each addition. Then mix in the vanilla extract until well combined.

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

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

Pour half of the batter into the prepared Bundt pan and spread it out evenly. Sprinkle the cinnamon-sugar filling over the batter, then top with the remaining cake batter.

Use a knife or skewer to gently swirl the cinnamon-sugar filling into the cake batter.

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

Once baked, cool the cake in the pan for 15-20 minutes, then invert it onto a wire rack to cool completely.

While the cake cools, whisk together the powdered sugar, milk (or heavy cream), and vanilla extract until smooth. Adjust the consistency with more milk if needed.

Drizzle the glaze over the top of the cooled cake, allowing it to drip down the sides.