Crack open 5 salted duck eggs and separate the yolks from the whites. Clean the yolks on running water. Place the yolks on a heat-proof plate or bowl.

Spray the yolks with a small amount of Shaoxing wine/baijiu/other alcohol to remove the fishiness.

Place the yolks on the preheated steamer and steam on medium heat for 8-9 minutes or until cooked.

Line a small freezer-friendly container with cling wrap. Transfer the mixture to the container. Freeze for 2-3 hours or until firm to touch.

Remove the frozen lava filling from the container. Use a knife to divide it into 12 pieces equally (12 gr each). Shape into balls and place them in a freezer-friendly airtight container. Make sure to work fast or the lava filling will melt.

Freeze the balls for at least 4 hours or overnight.

Transfer the mixture to a non-stick frying pan. Heat on low heat. Keep stirring the mixture with a spatula while cooking and cook until it thickens and turns into a dough. Don't overcook or it will turn dry and crumbly.

While the dough is still hot, cover with a cling wrap to prevent it from drying out. Once the dough has cooled completely at room temperature, store it in the refrigerator until using it.

Make water dough. Sift all-purpose flour, confectioners' sugar, and salt into a medium ball. Add butter and ice water. Mix with a spatula until a dough forms. Knead with your hands or mixer with a dough hook attachment until the dough turns smooth and elastic. To test the dough, stretch it into a thin semi-transparent sheet and it should hold well. Cover with a cling wrap. Rest the dough for at least 15 minutes.

Make oil dough. Sift cake flour into a medium bowl and add shortening. Mix and press it together with your hand until smooth. Cover with a cling wrap. Set aside.

Remove the custard filling from the refrigerator. Divide the custard filling into 12 pieces equally (27 gr each). Shape into balls.

Take a custard ball and make a deep well in the middle with your thumb. Insert one frozen lava ball into the middle and then close the top and reshape it into a ball. Repeat for the remaining pieces. Make sure to work fast or the lava filling will melt. Just take out 3 lava fillings from the freezer at one time to prevent the lava fillings from melting.

Freeze the filled custard back into the freezer until you are ready to assemble the rest of the mooncakes.

Divide the water dough into 12 pieces equally (15 gr each). Roll them into smooth balls. Cover the balls with a cling wrap to prevent them from drying out. Set aside.

Divide the oil dough into 12 pieces equally (13 gr each). Shape into balls. Cover the balls with a cling wrap.

Place a ball of oil dough on the working surface with the seam side up. Flatten the dough with a rolling pin, about 2.5 inches/6.5 cm in diameter. Place a ball of oil dough in the center and lift the edges of the water dough to cover the oil dough. Pinch the edges of the water dough to seal. Repeat with all pieces of dough.

Take a formed ball and place it on the working surface with the seam side up. Gently roll it into a long strip, about 5.5 inches/14 cm. Roll it tightly like a Swiss roll. Turn the dough vertically and roll it again into a long strip, about 5.5 inches/14 cm. Roll it tightly again. Cover and set aside. Repeat with the remaining balls.

Beginning with the first roll you made, take a roll and press in the middle so the swirled ends are pressed out. Then, fold the swirled ends into the center and press them down. Roll thinly with a rolling pin into 5 inches/12.5 cm in diameter.

Place the filling in the center. Lift and stretch the dough over the filling ball to cover completely. Pinch to seal. Place on the prepared baking sheet with the seam side down. Repeat with all the dough and filling.

In a small bowl, lightly beat the yolk. Brush a thin layer of the egg wash onto the tops of the pastry. Wait for 5 minutes and brush a second layer of yolk.

Wet your fingertip with water and dip it into the black sesame seeds so the seeds stick to your finger. Tap the top of each pastry to decorate.

Bake for 12-15 minutes or until the top is golden brown. Transfer the mooncakes to a cooling rack to cool completely.