Heat butter in a bowl and add chocolate, stir until the chocolate has completely melted.
Beat the eggs and sugar with an electric beater until light and thick (approx. 3-4 mins).
Sift the dry ingredients.
Add the dry ingredients as well as the chocolate/butter mix to the egg and sugar and beat until you have a smooth, thick batter.
Put it into the fridge for 30min-1hour, until it has formed a stiff enough cookie dough to shape into balls.
Pre-heat the oven to 180°C
Grease and line 2 large baking trays.
Spoon dollops of the cookie dough into your palms, roll into balls about the size of large marbles and place on the baking sheets at least 5 centimetres apart as they will spread as they bake.
Bake in the oven for 8-9 minutes – they should still be slightly gooey in the middle when you take them out of the oven.
Allow to cool slightly before taking off the baking paper and transferring to a wire wrack to cool completely.
To make the icing:
Using an electric mixer beat the butter and the icing sugar in bowl.
Lower the speed of the electric mixer until ingredients are combined - then up the speed to medium and beat until thick and light in colour.
Add the peanut butter and milk and beat for a further minute or two on medium until you have a smooth, soft, silky icing.
To assemble:
Make the cookie sandwiches by spreading a little of the icing onto half of the cookie
Seal them with the remaining half of the biscuits.