Line an 8-inch square pan with parchment paper, leaving an overhang on two sides. Melt 1/2 cup butter in a saucepan over low heat, then whisk in the granulated sugar and cocoa powder.

Slowly whisk in the lightly beaten egg and cook over low heat for about 3 to 5 minutes, stirring constantly, until slightly thickened. Do not let the mixture boil.

Remove the saucepan from the heat and stir in the vanilla, graham-cracker crumbs, coconut, and optional nuts. Press the mixture firmly into the prepared pan and chill for 20 to 30 minutes.

Beat the softened butter, cream or milk, custard powder, and confectioners sugar until smooth and spreadable. Spread this vanilla layer over the chilled base and refrigerate for another 20 to 30 minutes.

Melt the chocolate and 2 tablespoons butter together in short microwave bursts or over a double boiler, stirring until smooth. Spread the topping over the filling and add Halloween sprinkles while the chocolate is still soft.

Chill the assembled bars for at least 1 hour, preferably 2 hours, until firm. Let them stand at room temperature for 10 minutes, then cut with a warm, dry knife.