Mix the flour with the vanilla sugar and a pinch of salt.
Add the cold butter, cut in small pieces and mix until you obtain a sandy composition.
Add the egg, the vanilla essence and the sour cream.
Mix with your hands until you form a dough.
Don’t overwork the dough as you want tender, melt in your mouth cookies.
Split the dough into two balls, wrap in cling film, and leave in the fridge for 30 minutes to rest.
Meanwhile, mix the chopped walnuts with the plum jam.
Sprinkle flour on your work surface and your rolling pin.
Start by rolling the first ball of dough into a square or rectangular shape, very thin.
Cut the dough into squares.
Fill each square with a small quantity of the jam and walnuts mixture.
Fold the square, seal the edges, turn the pointy bit upwards and bend the corners, to resemble a half moon.
Preheat the oven to 180 degrees Celsius.
Bake the cookies on a tray lined with baking paper for around 15 minutes, until starting to get golden.
As you take them out of the oven, coat the cookies in powder sugar. Whilst hot, the powder sugar will stick to them so much better than cold.
Let the cookies cool down and serve with tea or coffee.