Prepare a surface with a piece of parchment paper, for spooning out the cookies at the end.
In a large saucepan, melt the butter over a medium heat.
Add the sugar, cocoa powder and milk to the melted butter, stirring until smooth. Continue cooking over a medium heat until the mixture boils. Allow it to boil for about 1 minute.
Remove from the heat and add the tahini. Stir this until the mixture is smooth.
Finally add the oats. Stir these to combine and then immediately begin dolloping out spoonfuls of the cookie mixture onto the prepared parchment paper. Work quickly as this will begin to set. This makes roughly 12-15 cookies depending on the size of the spoonfuls.
Allow to cool for about an hour, to allow these to firm up.