- Lightly grease a 9-inch (23 cm) square pan and line it with parchment paper so that it comes up the sides.
- For the crust, place the butter, sugar, cocoa powder and salt in a metal bowl and set over a pot of gently simmering water, whisking until the butter has melted. Add the lightly beaten egg and whisk until the mixture thickens to the consistency of pudding, about 1 minute. Remove the bowl from the heat and stir in the graham cracker crumbs and coconut (or almonds). Scrape the crust mixture into the pan and spread to level it. Chill the pan while preparing the filling.
- For the filling, beat the butter with 1 cup (130 g) icing sugar until smooth. Stir the hot chocolate mix, custard powder and salt with the milk (it will make a thick paste) and stir into the butter mixture until smooth. Beat in the remaining ½ cup (65 g) icing sugar. Do not overbeat—the filling should be smooth but not fluffy. Spread evenly over the crust (no need to refrigerate).
- For the topping, melt the chocolate and butter in a metal bowl placed over a pot of barely simmering water, stirring gently with a spatula until melted. Cool the chocolate slightly and then pour over the filling, spreading to cover it. Sprinkle the marshmallows on top of the chocolate in an even layer (it will not fully hide the chocolate) and, if you like, finish with a sprinkle of sea salt.
- Chill the pan for about 2 hours before slicing into bars. Nanaimo bars will keep in an airtight container in the fridge for up to 1 week.
I still include custard powder in my Nanaimo bar filling, even when I add the hot chocolate mix, to really capitalize on the creamy custard taste it adds.
Recipe from Baking Day with Anna Olson, Appetite by Random House, 2020