Preheat oven to 400°F (205°C). Line two baking sheets with parchment paper.

Unfold the puff pastry sheets on a lightly floured surface. Using a fork, dock the pastry all over to keep it from puffing into giant blisters. Trim to fit three even rectangles from the two sheets.

Place rectangles on the prepared sheets, brush lightly with the beaten egg, and sprinkle with granulated sugar (optional). Cover with another sheet of parchment and a second baking sheet to keep them flat while baking.

Bake for 12–16 minutes, until golden and crisp. Remove the top sheet of parchment and cool completely on a rack.

In a medium saucepan, heat milk over medium until it just begins to steam. In a bowl, whisk together sugar, egg yolks, and cornstarch until smooth.

Temper the eggs by slowly pouring about 1/3 of the hot milk into the yolk mixture, whisking constantly, then return the mixture to the saucepan.

Cook over medium heat, whisking until thickened and bubbling for about 1–2 minutes after it starts to simmer. Remove from heat, whisk in butter and vanilla.

Transfer to a bowl, press plastic wrap directly on the surface to prevent a skin, and chill until fully cold (at least 1 hour).

In a chilled bowl, beat the heavy cream, powdered sugar, and vanilla until soft peaks form. If using mascarpone, whip it until smooth and fold into the whipped cream.

When pastry and pastry cream are cold, cut each pastry rectangle horizontally if they puffed up. You’ll want three pancake-thin layers total.

Place the first pastry layer on your serving board. Spread (or pipe) a thin layer of pastry cream across it, then dot with fresh raspberries.

Add the second pastry layer, press gently, then add a layer of whipped cream and more raspberries.

Top with the final pastry layer. Warm the apricot jam and brush a thin glaze over the top pastry for shine, if desired. Dust with powdered sugar and garnish.

Chill the assembled dessert for at least 30 minutes to let layers settle. Slice with a sharp knife and serve.