Warm the whole milk in a saucepan until steaming but not boiling. Whisk the egg yolks, sugar, cornstarch, and salt in a bowl, gradually whisk in the hot milk, then return the mixture to the saucepan and cook over medium heat until thick and bubbling.

Continue cooking the pastry cream for about 1 minute after it begins to bubble, whisking constantly. Remove from the heat and stir in the chopped chocolate, butter, and vanilla until smooth, then cover the surface directly with plastic wrap and refrigerate for at least 1 hour.

Cook the raspberries, sugar, and lemon juice in a small saucepan over medium heat until softened. Stir in the cornstarch mixed with cold water and cook for 1–2 minutes, until slightly thickened, then cool completely.

Heat the oven to 400°F (200°C) and line a baking sheet with parchment paper. Cut each puff pastry sheet into six equal rectangles or squares, arrange the 12 pieces on the sheet, and chill for 10–15 minutes.

Beat the egg with the water and brush the tops lightly, avoiding the cut edges. Sprinkle with sugar if desired, then bake for 18–22 minutes, until deeply puffed and golden brown, and cool completely on a rack.

Slice each pastry horizontally and pipe or spoon chocolate pastry cream into the bottom halves. Add a small amount of raspberry filling, replace the tops, and finish with optional melted chocolate, powdered sugar, and raspberries.