1. Make the Raspberry Jam:

• Add the frozen raspberries, 100g granulated sugar, and 1 tablespoon lemon juice to a medium saucepan.
• Heat over medium heat, stirring occasionally, until the raspberries thaw, break down, and the mixture comes to a bubble.
• Once bubbling, reduce the heat to maintain a steady simmer. Let it simmer for 23-25 minutes, stirring occasionally and mashing down the raspberries with a rubber spatula, until the jam has thickened significantly.
• Test for thickness: Dip a cold spoon into the jam, then run your finger through the jam on the back of the spoon. If the line holds and doesn’t immediately run back together, it’s done.
• Pour the thickened jam into a shallow bowl to help it cool faster. Cover with plastic wrap (press it onto the surface to prevent a skin) and place it in the fridge to cool completely (at least 1 hour, or until room temperature or colder).

2. Make the Raspberry Syrup:

• While the jam cools, make the syrup. Place 100g granulated sugar, 120g water, and 30g frozen raspberries in a small saucepan.
• Bring to a boil over high heat, stirring until the sugar dissolves.
• Turn down the heat to a simmer and let it simmer for 3 minutes. Stir occasionally and gently break down the berries with a spatula.
• Strain the syrup through a fine-mesh sieve into a bowl, pressing gently on the solids to extract liquid. Discard the solids (seeds/pulp).
• If using, stir the 3 tablespoons of limoncello into the warm syrup.
• Let the syrup cool completely to room temperature.

3. Make the Mascarpone Filling:

• Ensure mascarpone cheese and heavy cream are cold, and jam/syrup are cooled.
• In the bowl of a stand mixer fitted with the whisk attachment (or using a hand mixer and large bowl), combine the cold mascarpone cheese, sifted powdered sugar, 2 tablespoons lemon juice, and vanilla paste.
• Whisk on medium speed until just combined and smooth, about 30 seconds. Scrape down the sides.
• With the mixer on low speed, slowly pour in the cold heavy cream.
• Once all cream is added, increase the speed to medium-high and whip until the mixture thickens and holds medium-stiff peaks (the peak holds its shape but the tip might curl slightly). Be careful not to overwhip into grainy territory.

4. Assemble the Tiramisu:

• Choose your assembly dish (approx. 8×10.5 inch or 9×9 inch).
• Spread a very thin layer of the mascarpone cream on the bottom of the dish (this helps anchor the ladyfingers).
• Briefly dip each ladyfinger into the cooled raspberry syrup – just a quick dip on each side (about 1-2 seconds total). Do not oversaturate them.
• Arrange the dipped ladyfingers in a single layer covering the bottom of the dish. Break cookies as needed to fit snugly.
• Spread half of the remaining mascarpone cream evenly over the ladyfinger layer.
• Dollop half of the cooled raspberry jam over the mascarpone cream and gently swirl or spread it slightly (it doesn’t need to be perfectly even).
• Arrange another layer of quickly-dipped ladyfingers over the jam/cream layer.
• Spread the remaining half of the mascarpone cream evenly over the second layer of ladyfingers, smoothing the top.

5. Chill Overnight:

• Cover the dish tightly with plastic wrap, pressing it gently onto the surface of the cream to prevent a skin.
• Refrigerate for a minimum of 8 hours, or preferably overnight. This chilling time is crucial for the ladyfingers to soften perfectly and the flavors to meld.

6. Finish and Serve:

• Once ready to serve, uncover the tiramisu.
• Gently spread the remaining half of the cooled raspberry jam over the top mascarpone layer in an even layer.
• Optionally, decorate with fresh raspberries and thin lemon slices.
• Slice into portions using a sharp knife, wiping the blade clean between cuts. Serve chilled.