Step 1: Prepare the Blackberry Mixture

1. In a small saucepan, combine the blackberries, sugar, and lemon juice. Cook over medium heat for about 5–7 minutes, stirring gently until the blackberries break down and the mixture thickens slightly.
2. Remove from heat and let it cool to room temperature.

Step 2: Make the Tiramisu Cream

1. In a large mixing bowl, whip the heavy cream until soft peaks form.
2. In another bowl, whisk together the mascarpone cheese, sugar, and vanilla extract until smooth. Gently fold the whipped cream into the mascarpone mixture until fully combined.

Step 3: Assemble the Tiramisu

1. In a shallow dish, combine the brewed coffee and coffee liqueur (if using).
2. Quickly dip each ladyfinger into the coffee mixture, ensuring they are soaked but not soggy.
3. In a rectangular dish, place a layer of soaked ladyfingers at the bottom. Spread half of the mascarpone cream over the ladyfingers, followed by half of the blackberry mixture. Repeat with another layer of soaked ladyfingers, the remaining mascarpone cream, and the remaining blackberry mixture.

Step 4: Chill and Serve

1. Cover the dish with plastic wrap and refrigerate for at least 4 hours, or overnight for best results.
2. Before serving, dust the top with cocoa powder and garnish with fresh blackberries.
3. Slice and serve chilled.