Line a 20 x 20cm tin with baking parchment. Melt the chocolate, butter and syrup in a heatproof bowl over a pan of just simmering water, stirring frequently until smooth
Crush up the biscuits until you have a mix of little bits and large chunks then add them to a bowl. Add crispy and chocolate M&M's to the bowl as well as the mini marshmallows.
Once the chocolate is melted, let it cool for a few minutes. Then pour it into the bowl with the biscuits and m&ms. Stir it all up ensuring that it is all coated in the chocolate mix.
Transfer to the tin, packing it in and smoothing over the surface. Press more M&M's in to the top.
Chill for at least 2 hours before cutting into 16 squares.