Passover lemon sponge cake roll

filling: 1/3 cup lemon juice 1/2 cup sugar 2 yolks plus 1 whole egg 1 stick (8 tablespoons) cold unsalted butter, cut into pieces
cake: 6 eggs, separated 1 cup plus 2 tablespoons sugar 1/4 cup potato starch 1/2 cup matzo cake meal pinch salt 1 teaspoon vanilla 2 tablespoons lemon juice
decorating & filling: 2 cups heavy cream 1/3 cup powdered sugar OR 1/4 cup sugar plus 2 tablespoons potato starch

additional equipment: a few tablespoons sugar or powdered sugar, wax paper, tea towel, berries for decoration

For the filling, stir together the lemon juice, sugar, egg, and yolks in a saucepan with a whisk. Place over medium heat and add the butter in small pieces while still whisking. Cook until it is thickened and just starting to boil (too much boiling will curdle it). Strain into a small bowl, and press saran wrap or wax paper onto the surface. Chill until completely cold.

Spray a rimmed cookie sheet or jelly roll pan that is about 11 x 15 x 1 inches with cooking spray, line it with wax paper, then spray the paper. Preheat the oven to 350 degrees.

Beat the egg yolks in an electric mixer on medium speed until broken up, then add the 1 cup of sugar and beat until light and thick. Scrape into another bowl, clean the mixer bowl, and beat the egg whites with the 2 tablespoons of sugar and the salt until they have stiff peaks but still look a little moist (if they are really stiff, you get a drier cake). Sprinkle the matzo meal and potato starch over the egg yolk mixture, and fold in, adding the vanilla, and lemon juice. Fold in the whites in two batches. Spread batter into the prepared pan. Bake for 8 - 12 minutes, until set.

Spread a tea towel on a large cookie sheet, and place a sheet of wax paper over it. Sprinkle the wax paper with sugar if really keeping strictly kosher for Passover, or powdered sugar if not. Unmold the cake onto the wax paper/towel and peel off the wax paper from the bottom. Using the towel to lift the cake, gently roll it up starting with a long side, rolling the sugar-sprinkled wax paper right in. Roll the towel around the whole thing, and leave it to cool to room temperature.

Whip the heavy cream with either 1/3 cup powdered sugar if not strictly kosher, or 1/4 cup granulated and 2 TBLS potato starch if really kosher. Fold together about half the whipped cream and all of the lemon curd to use for filling; reserve the remaining whipped cream for decorating.

Unroll the cake, spread it with the filling, and roll it back up. If it has cracked, don't panic, just kind of moosh it together, and all will be well -- you get to conceal the defects with whipped cream later. Put it on a serving plate and chill for a couple of hours or overnight if you have that long.

Just before serving, decorate the cake with the reserved whipped cream and berries.