Grease the base and sides of a 19cm x 29cm slice tray, then line with baking paper, allowing the sides to overhang. Combine the butter, golden syrup, chocolate and copha in a medium saucepan over low heat. Cook, stirring with a wooden spoon, for 5 minutes or until mixture is completely melted. Remove from heat. Place the Rice Bubbles and Twix in a large bowl. Add the mixture and stir until well combined. Spoon into slice tray and use the back of a spoon to smooth the surface. Place in the fridge for 30 minutes to set. Cut into bars to serve.