Preheat the oven and prepare the Bundt pan:
Preheat your oven to 350°F (175°C). Grease and flour a Bundt pan or use a non-stick spray with flour to prevent the cake from sticking.

Mix dry ingredients for the cake:
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

Cream the butter and sugar:
In a large mixing bowl, beat the unsalted butter and granulated sugar together until light and fluffy, about 3-4 minutes.

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

Alternate adding dry ingredients and buttermilk:
Gradually add the dry ingredients in three batches, alternating with the buttermilk. Begin and end with the dry ingredients. Mix until just combined.

Prepare the ribbon:
In a small bowl, mix the granulated sugar, brown sugar, and cinnamon for the ribbon.

Assemble the cake:
Pour half of the cake batter into the prepared Bundt pan. Sprinkle half of the cinnamon-sugar ribbon mixture evenly over the batter. Add the remaining cake batter on top and then sprinkle with the remaining ribbon mixture.

Swirl the batter:
Use a butter knife to gently swirl the cinnamon-sugar mixture through the cake batter, creating a marbled effect.

Bake the cake:
Bake for 45-50 minutes or until a toothpick inserted into the center of the cake comes out clean. Let the cake cool in the pan for 10-15 minutes before transferring it to a wire rack to cool completely.

Prepare the topping:
In a small bowl, mix together the granulated sugar and cinnamon for the topping. Once the cake is cooled, brush the top with melted butter and sprinkle the cinnamon-sugar topping over the cake.