Preheat the oven at 175 degrees Celsius.
Mix the butter in a bowl with an electrical mixer. Add the sugar, brown sugar, baking powder and the salt and mix together.
Add the egg and vanilla sugar and mix together with the electrical mixer.
Add half of the flour and mix it all by hand. Add the rest of the flour and make a compact cookie dough.
On a baking sheet place a baking paper and make small balls of the dough and place on the paper. Press gently in the middle of each cookie with a small spoon.
Place the baking sheet in the middle of the oven for 15 minutes or until the edges of the cookies are firm in consistence.
Take out the cookies and let them rest for 1 minute.
Pour evenly the dulce de leche in the middle of each cookie.
Place the walnuts on each cookie.
Transfer the cookies on to a wire rack and let them cool down.