To make the meringues:
Preheat oven to 200 degrees F. Line 2 baking sheets with parchment paper, set aside.
In the bowl of a stand mixer combine the egg whites, sugar and salt. It will be very thick and grainy.
Put the bowl over a double boiler and stir with a rubber spatula until the sugar is completely melted. This can take several minutes. You want to brush the sides down with the spatula to make sure all the sugar is melted and no grains are clinging to the sides.
Feel the egg mixture between your fingers to check for graininess. Once it is completely smooth, put it on your stand mixer and beat with the whip attachment on medium-high speed.
Beat it until it is light, fluffy, and glossy and the bowl feels just about room temperature. Add the scraped vanilla seeds and whip until they are evenly distributed.
Use a spoon to create mounds of the meringue on the parchment.
Wet the spoon and make a well in the center of the mound.
Bake the meringues for 1 1/2 hours. Turn off the heat and allow the meringues to sit in the oven for 30 more minutes. Remove from the oven and let cool completely before filling.
To make the curd:
In a bowl set over a double boiler, with an inch of simmering water below, whisk together the yolks, egg, sugar, lemon juice, zest and lavender.
Stir the mixture constantly until it starts to thicken, this can take several minutes. Add the butter and continue stirring until the butter is incorporated into the curd and it is thick enough to coat the back of a spoon. When you draw a line through the curd, it should be thick enough to stay put. Strain the curd into a container, cover with plastic (place it directly on the surface of the curd, to prevent a thick skin from forming) and place in the freezer for about 15 minutes or until cool, but not frozen.
Once it is cooled, fill the meringues with the lemon-lavender curd. Sprinkle with the lavender buds and serve.