Preheat the oven to 350°F. Grease and line an 8–9 inch Bundt pan with parchment paper if desired.

In a large bowl, beat the butter and sugar until light and fluffy. Add eggs one at a time, mixing well between each. Stir in the vanilla extract.

In a separate bowl, combine the flour and baking powder. Add the dry ingredients to the butter mixture in batches, alternating with the milk. Mix until smooth.

Divide the batter evenly into two bowls. Into one bowl, stir in the cocoa powder, cinnamon, and sugar to form the cinnamon swirl layer.

Layer the batters into the prepared pan, alternating between the vanilla and cinnamon batter. Use a knife or skewer to gently swirl the layers together for a marbled effect.

Bake for 45–50 minutes, or until a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for 10 minutes before turning it out onto a wire rack to cool completely.

Drizzle with melted milk chocolate before serving.