How to Make It
Make the crust: Heat the oven to 350° F. Butter a 9-inch square cake pan and line with parchment, leaving an overhang on two sides; butter the parchment.
Stir together the graham cracker crumbs, coconut, walnuts, cocoa powder, and sugar in a large bowl. Stir in the butter until evenly moistened, then add the egg until incorporated. With damp hands, press the mixture firmly into the bottom of the prepared pan in an even layer. Bake until golden brown, firm, and dry to the touch, about 15 minutes. Cool completely in the pan on a wire rack.
Make the filling: With an electric mixer on medium speed, beat the custard powder, vanilla, nutmeg, and 4 tablespoons butter in a large bowl until smooth. Add 1 cup confectioners’ sugar and beat on low until smooth. Scrape the bowl and add the cream; beat until smooth. Add the remaining confectioners’ sugar and beat until smooth. (The consistency should be spreadable. If not, beat in an additional teaspoon of cream.) Spread mixture evenly over the cooled crust and refrigerate until firm, about 1 hour.
Melt the chocolate and remaining tablespoon butter in a heatproof bowl set over a saucepan of simmering water, stirring until smooth. Spread the melted chocolate evenly over the chilled filling, then refrigerate for 30 minutes. With the tip of sharp knife, score the chocolate in thirds lengthwise, then into 1-inch strips crosswise. Refrigerate until very firm, about 1 hour. Holding the paper overhang, lift the layers out of the pan and transfer to a cutting board. Cut along the score marks to slice into bars.
Storage tip: The bars can be refrigerated in an airtight container for up to 3 days or frozen for up to 2 weeks.
A note on ingredients: Custard powder is available online and in specialty stores.