In a medium saucepan, combine 250 g raspberries with 50 g granulated sugar and 1 tablespoon lemon juice. Cook over medium heat, stirring occasionally, until the berries break down and the mixture thickens into a jam. Cool slightly, then refrigerate until cold.

In a small saucepan, mix the remaining 50 g sugar with 120 ml water and a handful of raspberries. Bring to a boil, reduce to a simmer, and cook 2–3 minutes until the sugar dissolves and the liquid is fragrant. Strain through a fine mesh sieve, pressing down to extract all the syrup, then let it cool completely.

In a large bowl, beat 450 g mascarpone cheese with 120 g powdered sugar, 2 tablespoons lemon juice, and 1 teaspoon vanilla extract until smooth. Add the cold heavy cream and whip on medium until the mixture is thick and holds soft peaks, being careful not to over‑whip.

Spread a thin layer of the mascarpone cream along the bottom of a 9x9 inch baking dish. Briefly dip each ladyfinger into the cooled raspberry syrup for 2–3 seconds per side, then arrange them in a single layer over the cream.

Spread half of the remaining mascarpone cream over the soaked ladyfingers, then add a generous layer of the cooled raspberry jam. Repeat the layers: syrup‑dipped ladyfingers, cream, and more jam. Finish with a final layer of cream and press lightly so the layers settle.

Cover the dish and chill in the refrigerator for at least 6 hours or preferably overnight. Top with additional fresh raspberries and a few lemon slices just before serving.