Preheat the oven to 350°F (175°C). Grease and flour a ring pan.

In a large bowl, cream together the softened butter and sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition.

In another bowl, whisk together the flour, baking powder, and salt. Gradually add this dry mix to your butter mixture, alternating with the milk, mixing just until combined.

Divide the batter between two bowls. In one bowl, stir in the cocoa powder and vanilla extract.

In the prepared ring pan, alternate layers of the plain and chocolate batters. Use a knife or a skewer to gently swirl through the batters.

Bake for about 30-35 minutes or until a toothpick inserted into the center comes out clean.

Let the cake cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.