Preheat the oven to 375F. Line a10-x-15-inch jelly roll pan with parchment paper, then grease and dust the parchment with flour, tapping our any excess.
Whisk the whole eggs, eggs yolk, 1/4 cup of the sugar and the lemon zest in a metal bowl placed over a pot of gently simmering water until it is just warmer than body temperature. Remove the bowl from the heat and whip the eggs on high speed until they have doubled in volume, about 4 minutes.
In a separate or clean bowl, whip the egg whites and salt starting on low speed until they are foamy, and then increasing to high, adding the remaining 5 Tbsp of sugar gradually. Continue to whip until the whites hold a medium peak when the beaters are lifted.
Sift the flour over the whipped whole egg mixture and fold in using a whisk, then fold in the whipped whites in two additions. Spread this in the prepared pan, being sure that the batter is level and even. Bake the cake for about 10 minutes, until it springs back when gently pressed.
Let the cake cool for about 2 minutes on a cooling rack, then loosen the sides with a spatula. Sift a layer of icing sugar over the surface of the cake and cover with a clean tea towel. Place a second cake pan over the towel and quickly invert the cake, removing the pan it was baked in. Peel off the parchment paper and dust this surface with icing sugar. Roll the two short sides of the cake in toward the centre with the towel (you can rest the cake on one side to get it to stay in place better) and let it cool this way to set its “memory” so the cake won’t crack once filled.
For the mousse, whip 1 cup of the whipping cream until it holds a soft peak. Stir the remaining ¼ cup of cream into the mascarpone cheese by hand, along with the lemon zest until the sugar has dissolved. Fold in the whipped cream and the lemon juice and chill until ready to assemble.
Carefully unroll the jelly roll cake and remove the tea towel. Spread the mousse over the cake, but leaving 2 inches at both of the short ends. Roll the short ends back up to the middle and press then in gently, to create a heart shape with the cake. You can gently press the centre of the cake down to create the point at the bottom of the heart. Wrap the roulade loosely with plastic wrap and chill it on its side, until ready to serve. When ready to serve, dust with icing sugar and slice the roulade and plate to show off the heart shape.
The roulade will keep, refrigerated, for up to 2 days.