Combine 12 oz raspberries, 1/4 cup sugar, and lemon juice in a saucepan over medium heat. Simmer for 5 minutes, stirring gently, until the sugar dissolves and the raspberries begin to break down. Pour half of this mixture into a separate bowl—this will be your dipping syrup for the ladyfingers. Return the remaining half to the saucepan and continue cooking for 10 minutes more, stirring occasionally, until it thickens into a jam-like consistency. This reduction concentrates the flavor and creates the perfect texture for layering. Remove from heat and let cool to room temperature.

Fill a saucepan with water and bring to a simmer. Place a heatproof bowl over the simmering water (making sure it doesn't touch the water) and add the 6 room-temperature egg yolks and 3/4 cup sugar. Whisk constantly for about 5 minutes until the mixture reaches 185°F on an instant-read thermometer—this temperature pasteurizes the eggs while creating a pale, voluminous base. This step is crucial for both safety and achieving the light, fluffy texture that defines tiramisu. Remove the bowl from heat and whip the mixture for another 5 minutes as it cools to approximately 75°F, using an electric mixer on medium-high speed. The mixture should be thick, pale, and tripled in volume.

In a separate bowl, whip 18 oz mascarpone and 1.5 cups heavy cream together for about 3 minutes until stiff peaks form. I prefer Galbani mascarpone because its thicker texture helps prevent the cream from becoming grainy. Be careful not to overwhip, as mascarpone can break if overworked. Gently fold this mascarpone cream into the cooled zabaglione from Step 2 until just combined—use a spatula and fold gently to maintain the airiness you've built into the mixture. Stir in the vanilla extract.

Working quickly so the ladyfingers don't become too soggy, dip each of the 20 Savoiardi ladyfingers into the reserved raspberry syrup from Step 1 for just 1-2 seconds per side—these biscuits are designed to absorb liquid quickly, so brief dipping is key. Arrange the dipped ladyfingers in a single layer in a 9x13 inch baking dish. Spread half of the cooled raspberry jam from Step 1 evenly over the ladyfingers, then spread half of the mascarpone cream mixture from Step 3 on top. Repeat with another layer of syrup-dipped ladyfingers, the remaining jam, and the remaining cream. Smooth the top layer with a spatula for an even finish.

Cover the assembled tiramisu with plastic wrap and refrigerate for at least 4 hours (or up to overnight if time allows). The chilling time allows the flavors to meld and the structure to set properly. Just before serving, dust the top generously with the powdered dried raspberries and scatter the 1.5 cups fresh raspberries over the surface for a bright, elegant finish.