Fit parchment into an 8-by-8-inch (20-by-20-centimeter) metal pan, leaving about 2 inches (5 centimeters) extending above two opposite edges. These exposed ends will form handles for lifting out the finished block.

Set a double boiler, or a heatproof bowl over boiling water, on the stove and whisk together the crust butter, granulated sugar, egg, cocoa, and vanilla. Once the butter melts, keep whisking for 1 to 2 minutes until the glossy mixture gains a little thickness, then take it off the heat.

Fold the graham crumbs, coconut, and chopped nuts into the warm cocoa mixture. All the crumbs should look moistened, with a sandy texture that holds together when pressed.

Pack the crumb mixture across the lined pan with your fingers, keeping the thickness even, then refrigerate for at least 15 minutes. The crust should feel firm before you cover it with buttercream.

Use a stand mixer’s paddle to beat the room-temperature filling butter at medium speed for about 1 minute, then blend in the cream and custard powder, scraping the bowl’s sides and bottom. The mixture should look evenly blended and pale yellow.

Mix in 1 cup confectioners’ sugar on low, followed by the remaining sugar, scraping underneath the mixture as needed, then beat at medium-high for 2 to 3 minutes. The finished buttercream should feel light and look fluffy and smooth.

Place spoonfuls of buttercream over the chilled crust, level them gently with an offset spatula, and refrigerate for about 30 minutes. The filling needs to set before it can support the chocolate layer.

Melt the topping chocolate and butter over low heat in a small heavy saucepan or double boiler, stirring frequently for 2 to 3 minutes, then remove from the heat. The glaze should look uniform and satiny, without unmelted chocolate pieces.

Pour the glaze over the chilled filling, promptly spread it edge to edge with an offset spatula, and add flaky salt if desired. A light touch helps keep the chocolate on top rather than dragging buttercream into it.

Refrigerate the pan for about 25 minutes to firm the topping. The chocolate should be hardened before you lift the block.

Lift the block using the parchment handles and divide it into 16 squares measuring 2 inches (5 centimeters) across. The cut edges should reveal a firm crumb layer beneath the creamy filling and chocolate cap.