Preheat oven to 200°C. Roast peanuts in oven till peel is dry, about 10 minutes. Remove 50 g and, when cool, rub off and discard peel. Set aside.
250 g peanuts
Bake remaining 200 g till brown and fragrant, another 5-7 minutes. Remove peel when cool and grind till fine-medium.
Mix plain flour, icing sugar and salt. Add 200 g ground peanuts. Mix thoroughly. Add peanut oil. Mix evenly, adding more oil if crumbly, or more plain flour if sticky.
200 g plain flour – sift with icing sugar, 150 g icing sugar, ¼ tsp salt, 140 ml peanut oil
Divide dough into small pieces weighing 10 g each. Roll each piece into a ball. Place balls on baking tray spaced about 3 cm apart. Garnish with remaining whole peanuts, 1 per ball, pressing nuts firmly into dough.
Preheat oven to 180°C. Line baking tray with parchment paper.
Place cookies on baking tray. Bake till peanuts on top are lightly browned, about 10 minutes.
Mix water and egg yolk thoroughly. Brush cookies with mixture.
2 tsp water, 1 yolk
Continue baking till cookies are well browned, 5-10 minutes. Transfer cookies to wire rack. Leave till cool. Serve immediately or store airtight.