Making the Cake:

1. Preheat the oven: Preheat to 350°F (175°C). Grease and line a 15×10-inch jelly roll pan with parchment paper, leaving an overhang for easy removal.

2. Beat the eggs: Using an electric mixer, beat eggs on medium-high speed for about 3-4 minutes until pale and fluffy. Gradually add sugar and continue to beat until well combined.

3. Add lemon: Stir in vanilla extract, lemon juice, and lemon zest until combined.

4. Mix dry ingredients: In a separate bowl, whisk together flour, baking powder, and salt. Gently fold the dry ingredients into the egg mixture in batches, careful not to deflate the batter.

5. Bake the cake: Pour the batter into the prepared jelly roll pan and spread it evenly. Bake for 10-12 minutes, or until golden brown and a toothpick inserted comes out clean.

6. Roll the cake: While the cake is baking, sprinkle a clean kitchen towel with powdered sugar. When the cake is done, invert it onto the towel and remove the parchment paper. Start at one short edge and gently roll the cake up with the towel inside. Let it cool completely.

Making the Blackberry Filling:

1. Cook the blackberries: In a small saucepan, combine blackberries, sugar, and lemon juice. Heat over medium for about 5-7 minutes until the berries break down.

2. Thicken the filling (optional): If desired, mix cornstarch with a bit of cold water and stir into the berry mixture. Continue to cook for 2-3 minutes until thickened. Remove from heat and let it cool completely.

Making the Whipped Cream:

1. Whip the cream: In a chilled bowl, beat heavy cream until soft peaks form. Add powdered sugar and vanilla extract, continuing to beat until stiff peaks form.

Assembling the Cake Roll:

1. Unroll the cake: Once the cake has cooled, carefully unroll it from the towel.

2. Spread the filling: Evenly spread the cooled blackberry filling over the surface of the cake, leaving a small border.

3. Add the whipped cream: Spread a thin layer of whipped cream over the blackberry filling.

4. Roll it up: Carefully re-roll the cake without the towel, starting from one end and rolling it tightly but gently.

Serving:

• Place the rolled cake seam-side down on a serving platter. Optionally dust with powdered sugar and garnish with extra blackberries or mint for decoration. Slice and serve chilled.