Melt your butter and chocolate together on medium-low heat in a medium/large saucepan, then once melted, add your Mornflake Beta³ Original Oat Bran Flakes and mix everything until completely combined.
Pour your mixture into a lined square baking tray, pressing it down firmly, then top with any decorations you fancy, refrigerating the cake for at least 2 hours to set.
Once set, slice into 16 pieces and store in a box for up to 1 month.