01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - Combine golden syrup, vegetable oil, and alkaline water in a bowl. Stir until thoroughly blended and smooth.
03 - Add flour to the wet mixture and mix until a soft, pliable dough forms. Cover with plastic wrap and let rest for 30 minutes at room temperature.
04 - Divide lotus seed paste into 12 equal portions, approximately 1.4 ounces (40 g) each. If using salted egg yolks, press one yolk into the center of each paste ball and reshape into smooth spheres.
05 - Divide the rested dough into 12 equal pieces, about 0.9 ounces (25 g) each.
06 - Flatten one dough piece into a thin disc. Place a filling ball in the center and gently stretch the dough around it, sealing completely. Repeat with remaining portions.
07 - Lightly dust each ball with flour. Place dough-side up into a floured mooncake mold and press firmly to imprint design. Invert mold and release onto prepared baking sheet.
08 - Bake for 5 minutes, then remove from oven and let cool for 10 minutes.
09 - Whisk egg yolk with water until blended. Brush a thin, even layer over the cooled mooncakes.
10 - Return mooncakes to oven and bake for 15 to 20 additional minutes, or until golden brown.
11 - Cool completely on baking sheet. Transfer to an airtight container and store for 1 to 2 days before serving to allow the pastry to soften and develop proper texture.