Add a heatproof bowl over a pot of gently simmering water, making sure the bottom of the owl isn’t touching the water. Add the butter, sugar and cocoa powder. Stir until the ingredients are melted and combined. Add the egg while stirring and continue mixing. The mixture will instantly thicken. Continue mixing for30-60 seconds. Take off the heat.
Add the digestives, coconut and slivered almonds to the bowl of a food processor. Blitz until all the ingredients reach a fine crumb. Pour the mixture into the chocolate mixture and use a spatula to stir until well combined.
Add the mixture to a lined 20 x 20cm square baking tin. Using clean hands, pat the mixture down firmly in the bottom of the tin and place in the fridge.
Add the butter, cream, custard powder, powered sugar and vanilla extract to a large mixing bowl and use an electric hand mixer to mix until well combined. About 2-3 minutes. Add the mixture to the baking tin and use a spoon to spread evenly. Place back in the fridge.
Add the cream and chocolate to a heatproof bowl and microwave for 30 seconds at a time, stirring each time until smooth. Pour over the chilled tin and spread around evenly. Chill for a final 1 hour and then use a large knife to slice into 3 x 3 squares before serving.
Nanaimo bars can be stored in an airtight container for up to three days.