Preparation

1. For the lemon curd: Fill a saucepan with an inch or so of water and bring to a simmer. Meanwhile, in a stainless steel bowl, whisk together the lemon juice, zest, sugar, and eggs. Place over the pan, making sure the bowl does not touch the water. Whisk constantly until thickened, 6 to 8 minutes. Remove from the heat, add butter, and whisk until smooth. Pour through a strainer into a clean bowl. Cool. Place a piece of plastic wrap directly on the surface and refrigerate until chilled.

2. For the sponge cake: Preheat oven to 204°C (400°F). Line a half-sheet baking pan (14 x 33 cm/18 x 13 inches) with a piece of parchment paper. Coat it with butter and dust with flour. Sift cake flour and baking powder together, and set aside.

3. Fill a saucepan with an inch or so of water and bring to a simmer. In a small saucepan, warm milk and add butter; stir until butter is melted. In the bowl of a mixer, combine the eggs and sugar. Place over the  simmering water and stir just until sugar is dissolved and mixture is warm to the touch. Beat until the mixture is thickened and tripled in volume, and falls in thick ribbons.

4. Sift the flour-baking powder mixture over the batter in thirds, folding gently after each addition.  Add warm milk and fold gently. Pour into the prepared pan, spreading evenly. Bake until the cake springs back lightly to the touch, about minutes. Remove from heat and allow to cool.

5. Lightly sprinkle the top of the cake with confectioners’ sugar to prevent sticking, then turn it onto a large piece of parchment paper. Carefully peel off the bottom parchment paper and place another clean piece of large parchment paper on top of the cake. Place the baking pan on top of the parchment paper, and flip the cake back over so the top is facing up.

6. For the berry filling: Place half the lemon curd into a large mixing bowl. Whisk the heavy cream to soft peaks, and fold gently into the bowl until it is mixed with the lemon curd.

7. Place the cake before you so the long edges form the top and bottom.  Spread the remaining lemon curd onto the sponge cake, stopping about 5 cm (2 inches) from the edge furthest from you. Now top the lemon curd with the lemon cream, stopping about 2.5 to 5 cm (1 to 2 inches) from all the edges. Sprinkle the lemon cream evenly with raspberries and blackberries.

8. Using the parchment paper to help control the process, carefully roll up the sponge cake tightly, applying very light even pressure. Roll the cake up in the parchment and place the cake seam side down. Wrap in plastic wrap and chill thoroughly, preferably for several hours.

9. For the mascarpone whipped cream: In the bowl of a stand mixer, beat together the cream and mascarpone until slightly thickened. Gradually add in the sugar and then Gran Marnier or other flavoring. Continue to beat just until very thick; you do not want it to clump. Keep covered and chilled until using.

10. To decorsate and serve: Carefully remove the plastic and parchment. Trim the ends and place on a serving platter. Put the whipped cream mixture into a pastry bag fitted with a large closed star tip, or other tip of your choice. Pipe stripes and swirls along the top of the cake, then the sides, then finish with a stripe along the ends of the cake without covering the cut ends. Refrigerate, serving as soon as possible, preferably within 3 hours.