In a medium saucepan, combine 2 cups of frozen raspberries, 1 cup of granulated sugar, and the juice of 1 lemon. Heat until bubbling, then simmer for 23-25 minutes, stirring occasionally until thickened. Cool completely.

In a small saucepan, combine 1 cup of water, ½ cup of sugar, and 1 cup of frozen raspberries. Bring to a boil, stir until sugar dissolves, then simmer for 3 minutes. Strain and cool. Optionally mix in 2 tablespoons of limoncello after cooling.

In a large bowl, beat together 1 cup of mascarpone cheese, ½ cup of powdered sugar, the juice of ½ lemon, and 1 teaspoon of vanilla paste until smooth. Whip 1 cup of heavy cream to medium-stiff peaks and gently fold into mascarpone mixture.

In your serving dish, spread a layer of mascarpone filling on the bottom. Dip ladyfingers quickly in raspberry syrup, then layer on top. Alternate layers of dipped ladyfingers, mascarpone cream, and raspberry jam, finishing with mascarpone cream. Smooth the top.

Cover with plastic wrap and refrigerate for at least 8 hours, ideally overnight. Garnish with remaining raspberry jam, fresh raspberries, and lemon slices before serving.