1. Preparation - Heat the oven to 180°C. Grease an 8-inch round cake pan and line the bottom with parchment paper. Sift together the flour and salt; set aside.

2. Orange Batter – Using a food processor, beat the butter at a medium setting, for 1 minute, until creamy. Add 100g of sugar and beat on a high setting, for about 3 minutes, until light and fluffy. Return to a medium setting, add the egg yolks 1 at a time until well mixed. Add in the orange juice, orange liqueur and orange zest until combined. Add in sifted flour gradually until just combined.

3. Egg Whites - In a clean mixing bowl, beat the egg whites on a medium speed until foamy. Increase to medium-high speed and gradually add 50g of sugar. Increase to high speed and continue to beat the whites to soft peaks. Do not overbeat. Fold in the egg whites into the orange batter until well blended.

4. Cocoa Batter - Pour half of the orange batter into a bowl. Add cocoa and fold very gently.

5. Assemble and Bake - Pour the cocoa batter into the prepared cake pan, followed by the orange batter. Bake until a toothpick or a skewer inserted in the centre comes out clean, for 40 to 45 minutes. Cool the cake on a wire rack and chill for at least 2 hours.

6. Serve - Cut the cake into slices and serve chilled.