Preheat oven to 350°F (175°C).
In a medium bowl, mix together the all-purpose flour and golden syrup until you get a sticky dough. It should be slightly crumbly but still hold together when pressed.
Add in grape seed oil and lye water and knead with your hands until it forms a smooth, pliable dough. If it’s too dry, you can add a bit more oil or water.
Divide the dough into 8 equal pieces and roll each piece into a ball. Put them on a parchment-lined baking sheet and press down lightly with your fingers to flatten them out. Bake for 10-12 minutes until golden brown.
Meanwhile, in a food processor, add pitted dates, peanut butter, oat milk, almonds and cashew nuts and process until you get a thick paste.
Remove the pastry from the oven and let cool for 5 minutes before transferring to a cooling rack.
Take one piece of pastry at a time and fill it with 1 tablespoon of date mixture. Pinch the sides together to seal the mooncake and press down lightly on top to flatten it. Place them back on the parchment-lined baking sheet.
In a small bowl, mash together 1/4 of a banana with 1-2 tablespoons of room-temperature water until you get an even paste.
Brush the banana paste onto each mooncake and return to the oven for 5-7 minutes until golden brown.
Remove from the oven and let cool completely before serving.