Add sugar and water into a small pot and bring to a simmer over high heat.

Let the sugar completely dissolve and then reduce to very low heat.

Add in the lemon and vinegar, cooking until the mixture reaches 105°C on a candy thermometer, about 10-20 minutes. Once the syrup reaches 108°C, remove from heat. Don't let the mixture go above 108°C.

Add in the baking soda (dissolved in 1 teaspoon water).

Let the syrup cool, and then strain into a clean and sterilized glass jar.

Store at room temperature for up to 6 months or longer.

In a bowl, mix together golden syrup, vegetable oil and lye water.

Add in all-purpose flour and use your hands to gently mix until combined. Be careful not to over mix.

Cover the dough with plastic wrap and let it rest for 1 hour, or overnight in the fridge.

Divide the lotus seed paste into 12 equal portions and roll into balls, about 35g each.

With your finger, indent the lotus seed paste ball and add ¼ of a salted egg yolk.

Enclose the salted egg yolk with the lotus seed paste and roll into a ball.

Divide the pastry dough into 12 equal portions, about 15g each.

Roll into round balls.

Place the dough ball in between plastic wrap and use a rolling pin to roll it out very thinly.

Add the lotus seed paste filling in the centre, and enclose with the pastry dough.

Roll the pastry with your palms into a smooth round ball.

Lightly dust the round ball in a small bowl with 1 tablespoon of all-purpose flour. Brush off any excess flour.

Place the dough into a 50g mooncake mold and gently press with your thumb to fill the corners.

Invert the mold onto a baking sheet lined with parchment paper.

Press down on the plunger twice (firmly) and then release the mooncake out of the mold.

Repeat with the remainder.

Before baking, lightly spray the mooncakes with water.

Let the mooncakes cool for 15 minutes.

In a small bowl, beat together egg yolk with 1 teaspoon water.

Lightly brush the mooncakes with egg wash (avoiding the indented areas to retain the imprinted design).

Wait 2 minutes for egg wash to dry and apply a second light coating of egg wash.

Remove the mooncakes from the oven and let them cool on a wire rack.

Store cooled mooncakes in an airtight container, lined with parchment paper.

Let mooncakes sit for 2 days at room temperature before serving.