For the whoopee pies: Position 2 oven racks in the upper and lower thirds of the oven and preheat to 375 degrees F.
Combine both kinds of chocolate and the butter in a medium heatproof bowl. Fill a medium pot about a third of the way with water and bring to a simmer. Place the over the simmering water and lower the heat. Melt the chocolate and butter gently together, stirring and scraping the sides to avoid scorching, until melted, then remove the bowl from the heat. Cool 2 to 3 minutes.
Whisk together the sugar, eggs and vanilla in another bowl. Set aside. Sift together the flour, cocoa powder, salt and baking powder in another bowl. Whisk in the egg and then the chocolate mixture. Do not overmix.
Spray 2 baking sheets with nonstick spray and drop the batter on them in large spoonfuls about 2 inches in diameter, leaving room between each to allow spreading.
Bake, rotating and switching the positions of baking sheets halfway through, until the whoopee pies puff up slightly and settle, 6 to 8 minutes. Allow to cool for 10 minutes on the baking sheet before gingerly lifting them off and transferring to a flat surface.
For the filling: Heat the milk in a medium saucepan until hot and add 3/4 cup toasted almonds. Turn off the heat. Allow the almonds to steep in the milk 25 to 30 minutes. Strain the milk and pour it back into the same pan.
Beat together the sugar, cornstarch, flour and egg yolks in an electric mixer fitted with the whisk attachment until thick, 3 to 4 minutes.
Pour some of the hot milk mixture over the eggs and blend. Pour the egg mixture back into the milk and cook over medium heat, stirring constantly with a wooden spoon, until the mixture thickens, 5 to 8 minutes. Stir in the almond extract and continue to cook until the mixture begins to bubble, then cook for an additional 2 minutes, stirring. Transfer to a bowl and cover with a layer of plastic pressed against the surface so the cream doesn't form a skin. Refrigerate 1 hour.
Sandwich some of the cream and the remaining 1/4 cup toasted almonds between chocolate rounds. Drizzle with melted chocolate if using. Serve immediately or refrigerate until ready to serve.
This recipe has been updated and may differ from what was originally published or broadcast.