Preheat the oven to 180°C (350°F). Grease and line a 20-22 cm (8-9 inch) cake pan with parchment paper. In a large bowl, cream the butter and sugar together until light and fluffy.

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

In a separate bowl, whisk together the flour and baking powder.

Gradually incorporate the dry ingredients into the butter mixture, alternating with the milk. Mix until smooth. Split the batter into two equal portions.

Into one portion, blend in the cocoa powder, cinnamon, and extra sugar until well combined. Pour a layer of plain batter into the prepared pan, followed by a layer of the cinnamon batter. Continue layering until all the batter is used.

With a knife or skewer, gently swirl through the batter to create a marbled effect. Bake 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 transferring it to a wire rack to cool completely.

Dust with powdered sugar or drizzle with icing if desired.

Slice the cake and serve. Best enjoyed with a cup of tea or coffee!