- Scald the milk in a heavy saucepan. Meanwhile, in a bowl blend the yolks with the sugar and flour.
- Add a little of the milk to the yolks and mix well before adding the remaining milk and returning it to the rinsed-out pan.
- Stir continuously until the custard comes to the boil. You may need to whisk it to ensure it is lump-free and smooth.
- Add the vanilla essence (or rum if desired).
- Transfer the custard to a piping bag while still very warm, then pipe it into each éclair through the hole you’ve cut to allow the shells to dry out.
- Start making the icing by first creating a sugar syrup. Dissolve the sugar in the water, then bring the mixture to the boil for 2-3 minutes.
- Place the icing sugar and cocoa in a bowl, then gradually add the syrup until a smooth, stiff icing is formed.
- The leftover syrup can be stored in the refrigerator.
- Beat in the oil to give the icing a shiny appearance, then spread over the eclairs. Serve and eat the same day.