I'm fairly sure that this cookbook is out of print but I swear by this recipe. That said I've added a few annotations of my own

1 lb. Granulated Sugar
2 oz. Cocoa
¼ pint Milk I always go for full fat milk
2 oz. Butter Salted butter
½ teaspoon Vanilla Essence but NOT Vanilla Flavouring and I tend to go for a full teaspoon

You may also need a non-stick saucepan, a 6 inch square baking tin, a mixing spoon and a Sugar/Candy Thermometer

1. Place the sugar in a pan with the milk and soak for at least an hour.
2. Add the cocoa and the butter. It advisable to sieve the cocoa because getting rid of the lumps can be a pain
3. Place over the heat and warm, stirring occasionally.
4. When the sugar has quite dissolved bring the mixture to the boil. This is important and you may have to be patient. You are looking for a smooth dark brown glossy liquid with no lumps of cocoa or granules of sugar. Try coating the back of a mixing spoon with the liquid running your finger along the spoon and tasting to detect granules.
5. Boil hard. I prefer to bring the temperature up slowly, it makes it easier to control
6. The mixture should bubble up in the pan and should be stirred sufficiently to prevent burning. Stir as little as you dare but don't let it burn. It will start smelling like treacle if it does.
7. Boil for about 15 minutes but the time varies according to the rate at which the mixture boils.
8. When the mixture begins to sink in the pan and to form a slight roughness round the sides it is cooked. I heat it to 119 C (246 F) using a Sugar or Candy Thermometer. This temperature is known as Firm Ball, most recipes tell you to go for Soft Ball 112 to 116 C (234 to 241 F) but I prefer the texture of Firm Ball.
9. Remove from heat and add the vanilla essence. Don't just turn the heat off, take the pan away from the heat. Wait for the mixture to settle down before adding the vanilla essence otherwise it evaporates too quickly when you mix.
10. Leave to cool slightly then stir and beat until thick. I beat it with my mixing spoon and not anything mechanical. The mixture can thicken quite suddenly and become difficult to pour.
11. Pour into oiled tins and cut into squares with a palette knife before set. I use a small amount of salted butter (enough for a thin clear layer) to coat the tins. The very, very slight taste of salt this causes works rather well.
12. When quite cold divide into pieces and keep in an airtight tin. This recipe is about 60 years old so you are allowed to use an airtight plastic container.