Step 1

Line an 8-inch (20 cm) square baking pan with parchment, or grease lightly with butter.

Step 2

Prepare bottom layer: In a double boiler whisk butter with granulated sugar, cocoa powder, and egg until melted and mixture thickens slightly.

Step 3

Remove from heat and stir in graham wafer crumbs, chopped walnuts and shredded coconut until the mixture is well combined and resembles wet sand.

Step 4

Firmly press mixture into the square baking pan, making sure to press it into an even layer. Transfer pan to the refrigerator to chill until firm, at least 15 minutes.

Step 5

Next, prepare the middle layer of buttercream: In a stand mixer fitted with a paddle attachment, cream butter on medium speed until whippy then add whipping cream and custard powder and mix until combined. Add 1 cup (250 mL) of confectioners’ sugar and mix on low until incorporated, then add remaining confectioners’ sugar and mix on medium-high speed until the icing is light and fluffy.

Step 6

Once the bottom layer is chilled, pull it from the fridge and later the buttercream on top of the bottom layer and spread it evenly on top using a spatula, making sure to create a layer as thick as the bottom layer. Transfer pan to the refrigerator to chill until buttercream is set; around 30 minutes.

Step 7

For the top layer: In a small saucepan or a double boiler, heat the chocolate and butter over low, stirring often, until melted and evenly combined. Spread the chocolate evenly over the buttercream using a spatula and return pan to the fridge to chill and the chocolate hardens; around 25 minutes.

Step 8

Score the chocolate top using a warm knife then cut it into 16 squares. Nanaimo bars can be kept in the refrigerator for a few days before serving and freeze well.