1. Preheat the oven to 180'C or equivalent temperatures and grease and line two baking trays with a butter wrapper and greaseproof paper.
2. In the large bowl, cream together the butter and sugars until light and fluffy. It is important that the butter is soft so that the sugar can mix in easily. Once they are creamed together, lightly whisk the egg in a small bowl before adding to the mix a little at a time so it doesn't curdle. Finally, sift in the flour and salt and mix with a wooden spoon until it comes together as a dough.
3. Roughly divide the dough into two with the sharp knife. Put one half in another small bowl and mix in the sultanas and oatmeal with your hands to distribute them as well as you can. Then roll into 9 walnut-sized balls, making sure there are a number of sultanas in each cookie and put on the two baking trays. Make sure to leave enough room between them as they spread during baking.
4. Press down each cookie with your fingers before putting them into the preheated oven for 7 to 8 minutes until light golden brown and still soft to the touch, as this will make them squidgy in the middle. While the oatmeal and sultana cookies are baking, prepare the chocolate by breaking into squares and cutting in half again.
5. When the oatmeal cookies have baked, take them out of the oven and leave to cool on the trays for at least 5 minutes. While they are cooling, mix the chocolate into the other half of the dough with a wooden spoon or your hands and roll into more walnut-sized balls. Carefully, transfer the oatmeal cookies to the wire rack with the fish slice before pressing down the chocolate ones with your fingers and baking for 7 minutes. Baking for 7 minutes creates a lovely squidgy cookie but if you prefer you can cook them for 10 minutes for a more biscuity texture. Trust the timings even if the cookies look less cooked than you would imagine as they continue to cook as they cool.
6. Take the trays out of the oven and leave the chocolate cookies to cool for 5 minutes on the tray. Then move gently to a cooling rack and leave to cool for 10 - 15 minutes till they have set and hardened on top. When you are ready to eat one of them, enjoy the oozing chocolate in the cookies while they are still warm or cool completely and enjoy as a lunchtime treat.