Lightly grease a 23 x 33cm baking tin, then line with a strip of parchment paper that overhangs the two long sides of the tin. Secure the paper in place with two metal clips.
To make the base, place the digestives or graham crackers into a large freezer bag and crush into crumbs using a rolling pin (a few small chunks are totally fine).
Place the butter, cocoa and sugar into a heatproof bowl set over a pan of simmering water (ensuring the bottom of the bowl doesn’t touch the water underneath) and stir until the butter is melted and everything is smooth.
Add the eggs and whisk for a couple of minutes, or until the mixture thickens. Remove from the heat and mix in the remaining ingredients. Scrape into the prepared tin and spread into an even layer. Refrigerate while you make the peppermint layer.
For the peppermint layer, place the butter into a large bowl and, using an electric mixer, beat on medium speed, or until the butter is light and creamy.
Slowly incorporate the sugar, then increase the speed to high and beat for about 5 minutes, or until light and fluffy.
Pour in the cream and peppermint extract, plus a drop or two of food colouring (if using) and mix until evenly combined. Scrape into the tin, spreading over the base in an even layer. Refrigerate for an hour before making the topping.
Place all the topping ingredients in a heatproof bowl set over a pan of simmering water (ensuring the bottom of the bowl doesn’t touch the water underneath) and stir occasionally until everything is melted and smooth.
Remove from the heat and pour over the filling, spreading into an even layer. Refrigerate until set.
To serve, use the parchment paper to lift the mixture from the tin, then cut into pieces using a sharp knife. If you want clean slices, using a knife that’s been heated in hot water (dried before using) for each cut will help.
Store in a sealed container in the refrigerator for up to a week.