Preheat oven to 350F and line 3 cookie sheets with parchment paper. Set aside.
In a bowl of a standing mixer, place the butter and sugar and mix until light and fluffy.
Add the egg and vanilla following the flour and cocoa powder. Mix in the chocolate chips. (and nuts if using)
Divide the dough into thirty 1" balls and make an indentation in the center of each cookie (you can use the end of a wooden spoon or your thumb). Fill it with 1/2 tsp of Dulche De Leche and bake for 12-14 minutes. Just until the Dulche De Leche starts to bubble.
Remove from oven and allow to cool completely.
Using a small spoon drizzle the cookies with melted chocolate chips.