Preheat the oven to 140 degrees celsius and prepare a baking tray by either lightly greasing it or lining it with silver foil.
With an electric mixer, cream the butter and icing sugar until glossy and completely combined. This usually takes 1-2 minutes. Beat in the vanilla essence.
In a separate bowl, combine the flour and custard powder. Lightly mix these with a spoon, then (with the mixer switched off) sift the combination into the butter and icing mixture.
Stir the flour and custard powder mix into the butter and icing sugar mix so that it is combined just enough so that you can mix it again with the electric mixer without huge clouds of flour being thrown from the bowl.
Add the cocoa powder as well, then continue beating with the electricc mixer for a further 1-2 minutes to completely combine all of the ingredients and make a uniform chocolate dough.
Use a sharp knife coated with flour to slice the turkish delight into small chunks. Place these chunks in the chocolate dough and fold them through until they are embedded into the dough.
Cover your hands in flour to prevent the dough sticking, then roll small balls of chocolate dough and place them onto the baking tray. Leave about 2 centimeters of space between each chocolate dough ball to allow for spreading. Dough balls of between 1 and 2 teaspoons worth of dough work well to create a nice sized biscuit, but you can make the cookies just about any size you want!
Use the heel of your hand to gently flatten each dough ball slightly.
Top each cookie with a chunk or two of extra turkish delight.
Bake for 15-20 minutes, or until cooked through. The easiest way to test this is to remove one of the cookies from the oven at the 15 minute mark while the others continue cooking and crack the cookie open. If the centre is still gooey, leave the cookies in for another 2-3 minutes and check again.