Preheat your oven to 200℉ and line two baking sheets with parchment paper.
In a medium saucepan, add 1 inch of water and bring it to a slight simmer.
In a heatproof bowl of a stand mixer, add egg whites and sugar. Whisk together until combined, then place over the simmering water, ensuring the bowl does not touch the water.
Whisk the mixture constantly until it reaches 140℉.
Remove the bowl from heat and attach it to your stand mixer with the whisk attachment. Start whipping the mixture on high speed until soft peaks form.
Add in the lemon juice, salt, and vanilla extract. Continue whipping until stiff peaks form.
Prepare your piping bags with a large star tip. Carefully spoon the meringue into the piping bag, twisting it closed at the top.
Pipe small dots of meringue under the four corners of the parchment paper to hold it down.
Pipe 2-inch rosettes onto the parchment, leaving about 1 inch between each.
Bake the meringues for 1 and ½ to 2 hours, until the outsides are crisp to the touch.
Once the pan is cool to the touch, gently remove the cookies. Store them in an airtight container or serve immediately.