• Edited: Preheat oven to 375 F (190 C) for a conventional oven. If using a convection oven, preheat to 350 F (180 C). Prepare egg wash by lightly beating the egg yolk with 1/4 tsp water. Set aside

If using roasted peanuts:

• I use store-bought skinless peanuts that are already roasted and unsalted. If you use raw peanuts, roast them in the oven at 350 F (180 C) for conventional oven, 330 F (165 C) for 8-10 minutes, stirring them halfway. Let them cool completely
• Place the roasted peanuts in a food processor and grind until fine in texture and the peanuts will feel "damp/wet" because some of the oil from the peanut has been released, but don't ground into a peanut butter consistency though

Prepare the cookie dough:

• UPDATE: I have recently tried without baking powder with a good result, so I have removed it from the recipe.
• Place ground peanut or peanut butter, sugar, flour, and salt in a large mixing bowl. Stir to mix with a wooden spoon. Then add half of the cooking oil (don't pour in all at once), and continue to gradually add the oil. Mix with your hands until you can form a dough that will not fall apart. If the dough feels dry, add a bit more oil, one teaspoon at a time.

Shaping the dough:

• Shape the dough into balls about 10 gr each. Place them on a cookie sheet lined with parchment paper about 1/4 inch apart, they don't expand much. Continue with the rest. 
• I made three different styles. You can use a straw to make a small indentation in the middle (mine are a bit too deep). You can also use a fork to gently press the dough ball down while making the crisscross pattern, or you can simply insert a half piece of peanut on top of the cookie dough

Bake the cookies:

• Brush the cookies with egg wash and bake for about 15-18 minutes (updated) on the middle rack. Rotate the pan halfway through baking, front to back so they bake evenly. They should be golden brown. If you make larger cookies, you may need a few minute extra
• Remove from the oven and let them cool for about 15 minutes before removing them to cooling rack to cool down completely. They are very soft when they are warm. Store in the air-tight container at room temperature for up to 1 month