Line a large tray with baking paper and preheat oven to 170C
Mix flour, bicarb soda and cocoa powder together in a large mixing bowl. In a separate bowl whisk together the syrup, vanilla paste, oil, alkaline liquid and egg yolk until combined.
Combine wet & dry ingredients until a smooth dough forms, adjust with a little extra flour if required until dough is pliable.
Rest dough for a 1/2 hour in the fridge.
Once the dough has rested for a sufficient time, roll into a log and divide dough into balls approximately 20g each in size for consistency and uniformity.
Roll out each ball into a flat disc. Use cling wrap between the rolling pin and dough as it might be a little sticky, dust sparingly with a little flour if needed.
Spoon a little of the Manjari filling into the disc and add a salted egg yolk in the middle (if using). Some bakers prefer to wrap the salted egg yolk individually in extra dough to keep the shape of the yolk, which is personal preference.
Shape the pastry disc into a neat ball, dust ball sparingly with flour.
Gently press the ball into the mooncake mould and remove, arrange cake on a baking tray and repeat the process until all the dough and filling is used.
Bake mooncakes at 170C for 10-15 minutes and remove tray from the oven. Mix the egg yolk with some water and brush over mooncakes. Return tray to the oven and continue to bake again for 10 minutes until cakes are golden brown.