Line an 8x8 inch dish with parchment paper or foil.
For the base
Melt the butter in a bowl over a pot of simmering water, making sure the water does not touch the bowl.
Remove from heat then whisk in the cocoa powder, salt, and sugar. Slowly drizzle the egg in while whisking. Return bowl to double boiler and mix over heat until thickened.
Fold in the graham cracker crumbs, shredded coconut, and almonds. Press into the bottom of your prepared pan and chill.
For the middle layer
For the filling add the room temp butter, powdered sugar, custard powder, and cream to your mixer fitted with a paddle attachment then beat on high until light and fluffy, about 4 minutes.
Spread onto the chilled base and smooth to the edges creating a level layer. Return to fridge to chill while you make the ganache.
To make the ganache
Melt the chopped chocolate and butter in a double boiler or microwave then spread onto the buttercream later and chill until completely set.
Once set, cut into bars using a sharp clean knife and serve. Store leftover bars in the fridge.
Notes
Make sure to line your baking pan so you can remove the bars easily.
A double boiler allows you have more control when melting chocolate and butter so they don't burn/split. To do this simply place a heat-proof mixing bowl above a pot of hot but not boiling water.
It's important to remove the melted butter from the heat before adding the egg so it doesn't scramble.
I chopped my almonds in the food processor to save time but you can also finely chop them or crush them in a plastic bag with a rolling pin.
A teaspoon of vanilla is a great addition to the buttercream.
Crush the Graham crackers in a plastic bag so there are some larger pieces for texture, don't let them get too fine and powdery.
You can add more cream for a lighter buttercream.
Powdered milk with a teaspoon of vanilla can be subbed for custard powder.
It's important to chill the bars so the layers don't mix into each other as they set.
Storage - the bars will keep well in the fridge for 3-4 days or can be frozen for up to 3 months. Thaw completely before serving.