For the pavlova: Preheat the oven to 200 degrees F and line 2 sheet trays with silicone pads.
Put the egg whites in the bowl of a stand mixer fitted with the whisk attachment and whisk just to break up the egg. Add the granulated sugar and mix on medium until it holds stiff peaks. Add the powdered sugar and salt and whisk on medium-high to tighten, about 1 minute. Scrape the bowl down with a spatula and whisk on high for 30 seconds just to make sure it's even in texture with glossy hard peaks.
Put the mixture into a disposable pastry bag and snip off the top inch. Pipe the egg white mixture on the lined sheet trays in six 3- to 4-inch filled-in circles, as if you are putting icing on top of a cupcake. Create a well on each pavlova using the bottom of a spoon, then bake until the pavlovas are hard to the touch, about 2 hours. Allow to cool completely at room temperature on a wire rack before serving.
For the lemon curd whipped cream: Meanwhile, put the juice, zest, yolks, granulated sugar and scraped vanilla seeds into a medium pot. Turn the heat on medium-high and cook, constantly stirring with a rubber spatula in a figure 8 motion until it coats the back of a spoon, about 10 minutes. Stir in the butter, salt and 1 cup cream until completely combined. Remove from the heat and pour through a fine mesh strainer into a shallow dish. Allow to cool for a minute or two before covering with plastic wrap, touching the curd to prevent a skin from forming. Put the dish in the refrigerator and allow to cool completely, about 30 minutes.
Whip the remaining 3 cups cream to medium peaks in the bowl of a stand mixer fitted with a whisk attachment. Fold the cooled lemon curd into the whipped cream and keep cold until ready to assemble.
When ready to serve, place the lemon curd whipped cream in a pastry bag and snip off just the top 1/2-inch. Put 1 small dollop on each serving plate. Place a pavlova on top and lightly press down into the lemon cream to prevent it from sliding. Pipe 1 large dollop of lemon curd whipped cream into the center of the pavlova where you created the well. Garnish with the berries on top of the whipped cream and on the plate, giving it a natural look, rather than an overly plated look. Dust with the powdered sugar.