In a medium saucepan, combine the frozen raspberries, granulated sugar, and lemon juice. Simmer over medium heat for about 23-25 minutes, stirring occasionally, until the mixture thickens and coats the back of a spoon. Remove from heat and cool in the fridge.

In a small saucepan, bring the water and granulated sugar to a boil. Add the frozen raspberries, reduce heat, and simmer for 3 minutes. Strain through a fine sieve and cool.

In a mixing bowl, combine the mascarpone cheese, powdered sugar, lemon juice, and vanilla paste. Blend until smooth. In a separate bowl, whip the heavy cream until medium-stiff peaks form, then fold into the mascarpone mixture.

Select a serving dish, dip ladyfinger cookies in raspberry syrup briefly, and layer them in the dish. Spread half of the mascarpone filling and half of the cooled raspberry jam. Repeat layers and finish with more mascarpone on top.

Cover tightly and refrigerate for at least 8 hours or preferably overnight.

Before serving, top with remaining raspberry jam and garnish with fresh raspberries and lemon slices. Cut into squares and serve.