Soften the butter and whip it with the sugar: mix it with awooden spoon until you get a soft and almost white foam.
Then add the eggs and the sifted flour. Mix gently and alsoadd the sifted cocoa powder. Add the dark chocolate chips (if you do not have the chocolate chips you can use some coarsely chopped chocolate bars with a knife).
Lastly add the yeast and stir to even out the mixture: cover the biscuit dough with plastic cover and let it rest in the fridge for about 40 minutes. Spread a sheet of baking paper on a baking sheet and with the help of two spoons make small balls well spaced apart.
Now bake them for 15 minutes in a preheated oven at 180 degrees: once cooked, remove the cookies from the oven. Let them cool for a few minutes. Serve still warm and with a light dusting of real sugar.