Put 1/2 cup of the peanuts in the bowl of a food proccesor. Add flour, baking soda and salt. Pulse until the peanuts are chopped really small.
In a saucepan, melt the butter over low heat. Set aside and cool to tepid.
Whisk in the peanut butter until smooth. Stir in the sugar and mix until smooth. add 4 of the egg whites and mix well.
Blend in the vanilla, using a wooden spoon, stir in the flour mixture in three additions mixing until just combined. Do not overmix.
Whisk the remaining egg white with the water in a shallow bowl. Place the remaining peanuts in a dish. For each cookie take around 1/4 of a cup.
Dip the top of each cookie into the egg white, then into the peanuts making sure to coat them generously.
Place the dough peanut side up on a lined cookie sheet and press the top gently with the heel of your hand.
Flatten into a disc around 8 cm diameter and 1 thick. Place the cookies 8 cm apart of each other. Sprinkle the top with sugar.
Bake on a previously pre-heated oven at 180°C for 16 minutes or until slightly brown and the tops are set.