Use a 30- × 11-cm loaf tin. Lightly grease and flour tin, tapping out any excess. Preheat oven to 180°C.
Place butter in a large mixing bowl; beat until creamy. Add sugar (reserve 1 tbsp) and essence and beat until well combined and pale and fluffy. Add eggs one at a time, stirring until well combined. Sift flour and baking powder onto a plate. Reserve 2 tbsp milk and stir the rest into the batter, alternating it with the flour. Spoon two-thirds of the batter into the prepared tin.
Stir sifted cocoa powder, 1 tbsp sugar and the last of the milk into the remaining cake batter and spread over the batter in the tin.
Twist a fork through the layers of batter to produce a marbled effect. Smooth the top and bake for 60–70 minutes until a skewer comes out clean. Turn cake out onto a wire rack to cool.