Melt the butter gently in a large casserole and add the flour. Stir to make a thick roux-cook this out for 3 minutes or so. Add the condensed milk and the sugar and stir to beat out the lumps. Once the sugar has melted you should have a nice smooth paste. Add biscuits, stir through until incorporated, then add chocolate. Stir until just melted and homogenous. Remove from heat to cool. Lump into a bag and pipe into the ice-cube holes with force.
Smooth over with a palette knife or something else flat.
Refrigerate until firm. Pop out the moulds and get involved!!