- Butter and line a 8×8″ square pan with parchment paper or aluminum foil, leaving an overhang on two sides for easy removal. Set aside.
- In a small saucepot, melt butter and whisk in sugar, cocoa powder, and salt. Remove from heat. Beat the egg well, and slowly pour in the egg while whisking vigorously. Return to heat and cook for 1-2 more minutes until the mixture is smooth and resembles hot fudge.
- Combine graham cracker crumbs, shredded coconut, and chopped nuts in a large bowl. Pour over the chocolate mixture and stir until well combined. The mixture might appear dry at first, but keep mixing, and it will come together.
- Transfer the mixture to the prepared pan. Press it firmly and evenly into the bottom of the pan and place it in the fridge to chill while preparing the next layer.
- In a large mixing bowl, beat the butter, custard powder, and vanilla until combined. Mix in 1 cup of powdered sugar and one tablespoon of cream at a time, mixing in between until smooth. Add 1-2 more teaspoons of cream, ONLY if needed, to reach a thick and spreadable consistency.
- Spread the buttercream evenly on the base layer using an offset spatula. Place in the freezer for 10-15 minutes to firm up while preparing the next layer.
- Melt the chocolate with the cream in a medium heatproof bowl over a saucepan of gently simmering water, stirring, until completely melted and smooth.
- Pour over the cold buttercream and quickly spread it all over using an offset spatula. Place back in the fridge until ready to cut into squares. Bars last at least a week in an airtight container in the fridge or freeze for up to 3 months. Enjoy!
TIP: For easier cutting, once the chocolate topping has set in the fridge, score lines for where you want to cut bards with a hot knife (knife dipped in very hot water, then dried). Place back in the fridge for another 20 minutes, then cut bars with the hot knife again, all the way through, whipping the knife when needed.