Once the cake has cooled gradually add it to a food processor and crumble until the cake mix resembles breadcrumbs.
Add butter, icing sugar and cocoa to a bowl with the crumbled cake and mix again until you have a firm dough, it should be similar in texture to shortcut pastry dough.
Prepare a plate or tray with baking paper and take a handful of dough at a time and mould around each creme egg until you have a fist sized ball. Place each egg on to the prepared tray/plate and put in the fridge to chill until firm.
Whilst they are chilling in the fridge grate 200g of chocolate into a bowl or plate large enough to roll the eggs in. Melt the remaining 400g of chocolate in another bowl over a pan of hot water, once melted leave it to cool slightly.
Once the dough balls are firm remove from the fridge and dip into the chocolate mixture making sure the entire surface is covered, then roll in the grated chocolate and place back on the tray to cool. Place your scotch eggs in the fridge for 10 minutes to help them set and then they are ready to serve.
Not that these need any more chocolate but if you wanted to help with the scotch egg theme further you could melt some white chocolate to make dips and colour them red & yellow to resemble tomato ketchup and mustard.