Heat the oven to 180C. 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 the Bosch MUM5 HomeProfessional Kitchen Machine, beat the butter in a mixing bowl at Setting 5 until creamy, for about 1 minute. Add 100g of sugar and beat at Setting 7 until light and fluffy, for about 3 minutes. At Setting 5, 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 at Speed 4 until foamy. Increase to Speed 5 and gradually add 50g of sugar. Increase to Speed 7 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.
Cut the cake into slices and serve chilled.