Make the raspberry dip for the ladyfingers. In a small bowl, whisk the raspberry jam with 1/2 cup water until smooth. Add 1 tablespoon lemon juice and, if using, the liqueur. Taste and adjust with a little more water or lemon to get a lightly tangy dip that’s not too thick.

Sweeten the raspberries. In a separate bowl, toss 2 1/2 cups raspberries with 2 tablespoons granulated sugar and the zest of half a lemon. Lightly mash about one-third of the berries with a fork to release juices. Set aside. Reserve the remaining 1/2 cup fresh raspberries for the top.

Whip the cream. In a chilled bowl, whip the heavy cream with 2 tablespoons granulated sugar and 1/2 teaspoon vanilla to medium peaks. Don’t overbeat—stop when it’s smooth and holds shape.

Make the mascarpone mixture. In another bowl, stir the mascarpone with the remaining sugar (about 1/4 cup), 1 teaspoon vanilla, and the zest of the remaining half lemon. Blend gently with a spatula until smooth. Fold in the whipped cream in two additions until fully combined and silky.

Set up your dish. Use a 9x9-inch (23x23 cm) square pan or a similar dish. A glass dish shows off the layers nicely.

Dip and layer the ladyfingers. Briefly dip each ladyfinger in the raspberry dip—just 1–2 seconds per side. They should be lightly moistened, not soaked. Arrange a snug layer on the bottom of the dish.

Add berries and cream. Spoon half of the sweetened raspberries (with their juices) over the ladyfingers. Spread half of the mascarpone cream on top, smoothing gently to the edges.

Repeat. Add another layer of dipped ladyfingers, the remaining raspberries, and finish with the rest of the mascarpone cream. Smooth the top.

Chill. Cover and refrigerate for at least 4 hours, or overnight for the best texture. Chilling allows the layers to meld and the ladyfingers to soften perfectly.

Garnish and serve. Right before serving, top with the reserved fresh raspberries. Dust lightly with powdered sugar and add a few mint leaves if you like. Slice into squares and serve chilled.