Combine 150 g low protein flour, 1¼ tsp of baking powder and ⅛ tsp fine salt, sift the flour mixture twice to incorporate and distribute the salt and baking powder evenly.
Coarsely chop 40 g walnut.
Pan roast chopped walnut with a dry pan over medium heat. Stir constantly with a spatula until fragrant or lightly brown. This recipe uses small amount of walnut, it should take about 2-3 minutes of toasting, set aside to cool.
Add walnut to a food processor/dry mill, mix with some flour mixture, about the same amount of flour from instruction 1. Adding flour will help keep the nut dry so that it can be ground finely without turning to paste.
Combine walnut with the flour mixture, press any lumps with a spatula to break them up, mix until well blended.
Combine 40 g salted butter, 40 g of oil, 100 g fine sugar and 30 g egg, whisk for about 1½ minutes until light and fluffy.
Mix in dry ingredients, fold with a spatula until dough forms.
Wrap the cookie dough with cling wrap and rest for 15 minutes.
Cookie dough is about 400 g, divide into 15 portions with 26 g each, arrange on a baking tray lined with parchment paper. Roll each dough into ball-shaped and flatten slightly, press the cookies dough with your thumb to create a hollow center.
Brush with egg wash.
Bake in preheated oven at 170°C (340°F) for 24 minutes. Allow the cookies to cool on a cooling rack for 1 hour after removing from oven.
Keep in an airtight container when the cookies are completely cooled, or you may heat seal the homemade walnut cookies in cookies packaging bag to keep them fresh.