- Preheat the oven to 325°F. Line the bottom of a 9-inch springform pan with parchment paper, but do not grease the pan.
- Whip the eggs and sugar on high speed until they are almost white in colour, more than triple in volume and hold a ribbon when the beaters are lifted, about 5 minutes. On medium speed, add the zest and juice.
- Sift the flour and salt and, still on medium speed, add the flour. Spoon about a cup of the batter into a bowl, and stir in the melted butter and vanilla. Add this all back to the full batter and stir until blended. Pour the batter into the prepared pan and bake for about 40 minutes, until the centre of the cake springs back when gently pressed. Cool the cake completely in the pan.
- For the mousse, whip the egg whites just until past foamy and set aside. Sprinkle the gelatin powder over ½ cup of water, stir and set aside. Bring the sugar and the remaining ¼ cup of the water to a boil and boil without stirring until it reaches 240°F on a candy thermometer. While whisking egg whites on medium speed, pour the sugar slowly and carefully down the side of the bowl and, once all added, increase the speed to high, whipping for 30 seconds to a minute, just until the whites hold a soft peak but are still hot. Beat in the gelatin until it has dissolved into the whites, then continue to beat until the mixture cools to room temperature. Whisk in the strawberry purée.
- In a separate bowl, whip the cream until it holds a soft peak and add the vanilla. Fold this into the strawberry mixture using a whisk, and folding it in 2 additions.
- To assemble, run a palette knife round the inside edge of the sponge cake pan and remove the cake, peeling away the parchment paper. Wash the pan, reassemble and place the cake layer back in the bottom of the pan. Slice the hulled strawberries in half and place them on the cake so that the flat side of each berry is pressed up against the ring of the pan, and they are very close together. Pour the mousse over the cake and berries and chill until set, at least 4 hours. Top the cake with any remaining strawberry halves right before serving.