Preheat oven to 180C / 355F (not fan-assisted). Line a baking sheet with baking parchment. Set aside.
Sift flour, icing sugar and salt into a large mixing bowl. Add in peanut butter and oil. Stir with a rubber spatula or with your hand until a dough is formed.
Scoop a spoonful of dough and roll into 1-inch balls or scoop 10 grams of dough and roll into a ball. Repeat until all the dough is used up. Place them 1 ½-inch apart from each other on the baking sheet.
Combine egg yolk and water together in a small bowl. Brush the top of the dough with egg wash, making sure they do not drip down to the bottom of the dough, otherwise the bottom may burnt during baking.
Bake the cookies for 12 to 15 minutes until the cookies crack and the top is golden-brown. Remove the baking sheet from the oven and let the cookies cool completely on the baking sheet before storing them in an airtight container (or in your belly!)
Fellow foodies who have tried the recipe feedback that the cookies are too sweet. I've not tested the recipe (sorry for procrastinating), but I will definitely update on this once I've done so.