Prepare Raspberry Mixture:In a bowl, mix raspberry jam, lemon juice, and half of the raspberries. Lightly mash to combine. Set aside.
Make the Coffee Soak:In a shallow bowl, combine brewed coffee with raspberry liqueur (if using). Let cool completely.
Whip the Cream:In a separate bowl, beat heavy cream until soft peaks form.
Make the Mascarpone Filling:In another bowl, whisk together mascarpone cheese, powdered sugar, and vanilla extract until smooth. Gently fold in the whipped cream until well combined.
Assemble the Tiramisu:Quickly dip ladyfingers in the coffee mixture and line the bottom of a dish (8x8 or similar). Spread half of the mascarpone mixture over the ladyfingers. Spoon half of the raspberry mixture on top.
Repeat Layers:Add another layer of dipped ladyfingers, then the remaining mascarpone mixture, followed by the rest of the raspberry topping.
Chill:Cover and refrigerate for at least 4 hours (or overnight) to let flavors meld and dessert set.
Serve:Garnish with fresh raspberries, white chocolate shavings, or a dusting of powdered sugar before serving.