Heat the oven to 350°F (175°C). Line an 8-inch square pan with parchment paper, leaving enough overhang on two sides to lift the chilled dessert out later.

Combine the graham-cracker crumbs, melted butter, granulated sugar, and pinch of salt. Press the mixture firmly and evenly into the prepared pan, then bake for 8–10 minutes and cool completely.

Whisk the sugar, cocoa powder, cornstarch, and salt together in a saucepan. Whisk in the egg yolks and whole milk until the mixture is smooth and no dry pockets remain.

Cook the mixture over medium heat, whisking constantly and paying attention to the corners of the saucepan. Once it thickens and reaches a gentle bubbling stage, continue cooking for about 1 minute.

Remove the saucepan from the heat and stir in the butter and vanilla extract. If the custard has any lumps, strain it before pouring it over the cooled crust.

Pour the hot chocolate filling over the crust and smooth the surface. Press plastic wrap directly against the filling to prevent a skin from forming, then refrigerate for at least 4 hours until fully firm.

Whip the cold heavy cream with the confectioners' sugar and vanilla until medium-stiff peaks form. Spread or pipe the cream over the chilled chocolate layer without pressing too firmly.

Scatter chocolate shavings over the whipped cream. Lift the dessert from the pan, cut it into 16 squares with a sharp hot, dry knife, and wipe the blade between cuts for tidy edges.