Melt the butter in the microwave and mix with the sugar and eggs to make a smooth paste. In a separate bowl, add the hazelnut drink to 50 g crushed pistachios and stir well. Combine the contents of the two bowls and beat to make a smooth mixture.
Add the sifted flour and beat just enough to blend all the ingredients together without beating in too much air.
Pour the mixture into the muffin cases to halfway up. Add two squares of chocolate to each case and then ¾ fill with the mixture (to leave enough room for the muffin to rise).
Preheat the oven to 180 ºC and then bake for 20 to 25 minutes.