Blend the butter and sugar on high speed for 5-7 minutes or until the sugar has dissolved completely.
Add the egg yolk and vanilla essence and continue mixing.
Stir in the sifted flour and grated walnuts. You will get a crumbly dough. Shape it into a ball and refrigerate for 30 minutes.
Preheat the oven to 180°C/356° F and line a large baking pan with parchment paper.
Once chilled , transfer dough to a floured surface and roll it out into a 0.5 cm (1/5 of an inch) thick sheet.
Cut out 4-6 cm (1 ½ - 2 ½ inch) diameter circles and transfer them to a baking pan lined with parchment paper.
Bake the cookies for 12-15 minutes or until the edges are lightly brown.
Allow the cookies to cool completely before filling with cream.
For the chocolate cream:
Put the sugar and water in a saucepan. Melt the sugar over medium heat, then add the chopped chocolate.
Take the saucepan off the heat, let it cool for a few minutes, then add the egg yolk and mix well with a whisk. Allow the cream to cool completely before proceeding to the next step.
Blend the butter for 1 minute, then fold in the chocolate cream, mixing constantly.
For the chocolate glaze:
Melt the chopped chocolate in a steam bath and add two tablespoons of oil. The oil will give the glaze a very nice shine.
Assembly:
Cover half the cookies with the chocolate glaze. Place the cookies on a rack over a cooking sheet and spoon the glaze onto the center of each cookie, making sure it covers the entire surface and the sides. Place a half walnut in the middle of each one.
Place the cookie rack in the freezer for a few minutes until the glaze hardens completely.
Pour the chocolate cream into a piping bag and pipe it onto the bottom side of the unglazed cookies.
Place a glazed cookie on top of a creamed cookie and decorate with walnuts.
Serving and storage:
The cookies can be served immediately after baking or can be stored for a few days in the fridge in a food contained with a lid.