In a saucepan, whisk the eggs with the sugar, lemon juice and zest. Stir constantly over moderate heat until thickened but still pourable, about 6 minutes. Transfer to a blender and let cool for 5 minutes. Add the butter and salt and blend until pale, about 1 minute. Transfer the lemon cream to a bowl, cover and refrigerate until cold.
Preheat the oven to 350°. Line a 12-cup muffin pan with paper liners and set 2 foil muffin cups in a pie plate. In a small bowl, whisk the egg whites with 2 tablespoons of the milk and the vanilla. Whisk in the remaining milk. In another bowl, whisk the cake flour with the baking powder.
In a bowl, beat the butter, sugar and salt until fluffy. Add the dry ingredients in 2 batches, alternating with the milk mixture. Spoon into the muffin pan and foil cups and bake for 20 minutes, until golden and springy when lightly touched. Let cool in the pans for 5 minutes, then transfer to racks to cool completely.
Slice off the top of each cupcake and spread with the raspberry jam. Replace the tops. Spread the lemon cream on top, garnish with berries and serve.
These cupcakes call for cake flour rather than all-purpose. Cake flour has less gluten protein than all-purpose, making for a lighter, springier cake. If you don’t have cake flour, replace 7/8 the amount called for with all-purpose flour; in this case, it would be 1 2/3 cups of all-purpose flour instead of cake flour.