Grease and line an 18cm square tin. In a bowl over a pan of barely simmering water, melt 200g of the chocolate and add three quarters of the tin of condensed milk and 20g of the butter (do not allow the bowl to touch the water).
Whilst the chocolate is melting, crush the biscuits with a rolling pin in a bowl (or blend in a food processor) until you have a mix of crumbs and small chunks. Add the marshmallows to this bowl.
When the chocolate is melted and combined with the butter and condensed milk, use a spatula (it will be a fairly thick mixture) to scrape the contents into the bowl containing the crushed biscuits and mallows. Stir thoroughly and press into the tin.
Leave to cool and set whilst you make the peanut butter layer. In a clean bowl over a pan of simmering water, combine the peanut butter and the remainder of the condensed milk, along with a dash of milk until a spreadable yet still thick consistency is reached. Keep adding milk if it is too thick to spread.
Spread this peanut butter layer onto the base (don't worry if it has still not cooled completely) and then melt the dark chocolate in a pan over simmering water or in blasts in the microwave and use a spatula to spread on top of the peanut butter layer.
Leave to cool and cut into slices. Eat now or if you can resist put in the fridge to chill and firm up.
This came about as a way to use chocolate, peanut butter and condensed milk all in the same recipe. Experiment with the base layer ingredients - you can use a combination of milk and dark chocolate. Also try adding chopped nuts or dried fruit.