Ingredients

115 g butter at room temprature

100 g of sugar
1 egg yolk, lightly beaten (large size)
1 teaspoon of natural vanilla extract vanilla powder
180 g flour
A pinch of coarse salt

Cover with Clingfilm and refrigerate for 2 hours.
Preheat the oven to 160 °.
Mix the cocoa nibs and 2 tablespoons of sugar in a bowl.

Divide the dought into small balls and dip them in the beaten egg white, then in the cocoa nibs.

Place on a baking sheet lined with parchment paper. 

Press down the center of each ball with your thumb or with the handle  of a wooden spoon.
Bake for 10 min. Remove the cookies from the oven and press down the centres if necessary.
Bake another 10 minutes until browned. Cool for few minutes before filling the cavity with jam.