In a saucepan, combine frozen raspberries, granulated sugar, and lemon juice over medium heat. Stir continuously until bubbling, then reduce to a simmer for around 25 minutes until thickened. Let cool completely.

In a small saucepan, combine sugar, water, and a cup of raspberries. Bring to a gentle boil, then reduce to a simmer for about 10 minutes. Strain the syrup to remove seeds and let cool.

In a large mixing bowl, beat mascarpone, powdered sugar, lemon juice, and vanilla paste until mixed. Fold in whipped heavy cream until medium-stiff peaks form.

Layer half of the ladyfingers dipped in raspberry syrup in a rectangular dish. Spread a layer of mascarpone filling, add half of the raspberry jam, and repeat. Finish with mascarpone filling. Cover and refrigerate for at least 8 hours.

Before serving, top with remaining raspberry jam and garnish with fresh raspberries or lemon slices. Serve chilled.