While inspired by the French pastry-case favorite, mille-feuille, this version is nowhere near as complex. We deconstructed all the key elements of the original to turn that beautiful, delicate dessert into a treat you can easily make at home. Store-bought puff pastry is baked between two baking sheets, preventing it from rising unpredictably, and yields jagged shards of crisp-tender pastry. Pseudo pastry cream, made from heavy cream and mascarpone and flavored with vanilla and lemon, whips into soft peaks in seconds. Tossing peak-season blackberries in a little lemon juice and salt tames their sweetness and draws out their juiciness. Combine all three components and they re-create the effect of eating a just-assembled mille-feuille, with a fraction of the effort. If you are using particularly tart berries, try starting with just half of the lemon juice.