In the bowl of a stand mixer, combine the softened butter and icing sugar. Beat the butter and sugar together with a paddle attachment until light and fluffy, about 2 minutes. Add the flour and salt and mix until the mixture is dry and crumbly. Add the whisked egg and mix just until the mixture begins to clump together.
Dump the dough mixture onto the counter and use your hands to press it together. Cover with plastic wrap and chill for at least 1 hour.
Preheat the oven to 375° F. Spray 9 individual 4-inch tart pans with non-stick spray. Alternatively, use a 10 inch tart pan with removable bottom. On a lightly floured counter, roll out the tart dough to 1/6 inch thickness.
Cut the dough into circles slightly larger than the tart pans. Line the pans with the dough, pressing to fix any cracks. Cut the dough off around the top of the ring and prick the dough several times with a fork. Transfer the tart pans to a baking sheet. Gather the scraps of dough together and gently re-roll to make the remaining tart shells.
Line each tart shell with a piece of foil and add dried beans or pie weights. Bake the tarts for 15 minutes. Remove the foil and pie weights. Bake for another 5 to 7 minutes until the tart shells are lightly browned and cooked through. Let cool briefly and then remove from the tart rings to cool completely.
Place a fine mesh strainer over a bowl and set aside. Cut the butter into small pieces and have it ready by the stove. In a small cup or bowl, combine the plain gelatin and water. Set aside to allow the gelatin to bloom.
In a small saucepan combine the sugar, egg yolks, lemon juice, 1 tbsp elderflower liqueur, and the lemon zest and whisk until completely combined. Place over medium heat. Bring to a simmer while stirring constantly. Allow the mixture to bubble a few times before removing from the heat.
Add the gelatin and whisk until it has fully melted. Whisk the cold butter into the lemon mixture until it melts. Add the remaining elderflower liqueur.
Pour the mixture through the strainer into the bowl. Place a piece of cling film directly on top of the lemon mixture to prevent a skin from developing. Chill the curd for 1 to 2 hours until it is cool and beginning to set.
Spoon the lemon filling into the tart shells and chill until firmly set.
Add the egg whites and cream of tartar to the bowl of a stand mixer fitted with a whip attachment and have ready. In a saucepan, combine the sugar and water. Place over medium high heat and stir briefly to dissolve. Bring the syrup to a boil without stirring and cook until the syrup reaches 230° F.
When the syrup reaches 230° F, turn on the mixer and beat the egg whites to soft peaks. Continue cooking the syrup until it reaches 240° F. Turn the mixer down to medium speed and slowly pour the syrup into the egg whites in a thin stream. Pour the syrup along the edge of the bowl and avoid the whisk which could fling the hot syrup out of the bowl.
Once all the syrup is added to the egg whites, turn the speed back up to high. Continue to whip the egg whites until they form stiff peaks and have cooled to room temperature. This will take about 7 to 10 minutes. Turn the mixer down to low and add the elderflower liqueur. Beat just until the liqueur has been fully incorporated.
Transfer the meringue to a large piping bag fitted with a star tip. Pipe the meringue decoratively on top of each tart. Use a kitchen torch to lightly brown the meringue. Alternatively, you can use the broiler in the oven to brown the meringue. Enjoy!