Rinse 2 cups of fresh raspberries and place them in a medium pot with 1/2 cup of water. Cook over low-medium heat for 3-5 minutes until the raspberries soften and release their juices. Mash the mixture with a fork, then strain it through a fine sieve to extract the juice. Return the liquid to the pot, adding 1/4 cup of sugar and 1 tablespoon of fresh lemon juice. Boil for a couple of minutes, then reduce to a simmer until thickened. Cool completely.

In a large mixing bowl, whip 1 cup of heavy cream using an electric mixer until stiff peaks form, which should take about 3-5 minutes. In another bowl, combine 8 ounces of cold mascarpone cheese, 1/4 cup of sugar, and 1 teaspoon of vanilla extract until smooth. Gently fold the whipped cream into the mascarpone mixture in thirds, taking care not to overmix. Slowly add in the cooled raspberry juice, folding gently until smooth.

Grab a shallow dish or trifle bowl. Quickly dip each ladyfinger into the cooled raspberry juice for just 2-3 seconds on each side to avoid sogginess. Arrange a layer of soaked ladyfingers at the bottom of your dish, ensuring complete coverage. Spread half of the raspberry mascarpone filling over the ladyfingers in an even layer. Repeat with another layer of dipped ladyfingers and top with the remaining filling, smoothing the surface.

Cover the assembled Raspberry Tiramisu tightly with plastic wrap or a lid. Chill in the refrigerator for at least 6 hours, though allowing it to set overnight is best.

In a small pot, combine 1 cup of raspberries and 1/4 cup of water. Cook until the raspberries break down, blend until smooth. Strain through a sieve to remove seeds, returning the liquid to the pot. Add 2 tablespoons of sugar, 1 tablespoon of lemon juice, and 1 tablespoon of cornstarch mixed with a little water. Bring to a boil and stir until thickened, then remove from heat and mix in 1 tablespoon of unsalted butter.

Once chilled, pour the cooled raspberry sauce over the top. Garnish with additional raspberries and mint leaves. Serve immediately or keep chilled until ready to present.