Preheat the oven to 200℉ and line two baking sheets with parchment paper.
Add 1 inch of water to a medium saucepan and bring to a slight simmer.
In a heatproof bowl of a stand mixer, add the egg whites and sugar. Whisk together until combined and place over the simmering water, ensuring the bowl does not touch the water.
Whisk the egg whites and sugar constantly until the mixture reaches 140℉. Remove from the heat and fit on the stand mixer with the whisk attachment. Whip until soft peaks form.
Add the lemon juice, salt, and vanilla extract. Whip until stiff peaks form. The meringue will begin to pull away from the sides of the bowl and when the whisk is removed, the peak will stand straight up. This can take a good 10-15 minutes of whipping at high speed.
Prep your piping bags by fitting with a large star tip. Carefully spoon meringue into the piping bag and twist closed at the top, being careful to not overfill the bag (fill ½-⅔ of the way full).
Pipe 4 small dots of meringue under the four corners of the parchment paper and press down. This will prevent the parchment from blowing up and sticking to your meringue cookies as they bake. Pipe 2 inch rosettes onto the parchment, leaving about 1 inch between them. Sprinkle with sprinkles, if desired.
Bake for 1-½ to 2 hours, until the outsides of the cookies are crisp to the touch. Turn off the oven and let the pan cool in the oven. Once the pan is completely cooled to the touch, remove from the oven.
Gently remove the cookies from the pan.. Store in an airtight container or serve immediately. Do not refrigerate or leave uncovered otherwise you risk the meringue becoming tacky and weeping.