Place the chocolate and butter in a heat proof bowl and place over a saucepan of simmering (not boiling) water. Stir occasionally and when melted remove and stir until smooth. Add the condensed milk, mix to combine and leave to cool for 5 to 10 minutes.
Preheat the oven to 160 celsius (320 Fahrenheit) and line 3 baking trays with baking paper. Place the flour, baking powder, salt and cocoa in a bowl and stir until combined. Add the chocolate and condensed milk mixture and mix until a smooth dough is formed. Add the chocolate chips and work through the mixture.
Take teaspoon amounts of dough and roll into a ball. Flatten between your hands and place on the tray, spacing them well to enable the cookie to rise and spread. Bake for 12 to 15 minutes and leave on the tray for a minute to firm up before placing on a wire rack to cool completely.