Preheat the oven to 250°F. Line a baking sheet with parchment paper and set aside.
In the bowl of a stand mixer fitted with a whisk attachment add egg whites and beat on medium speed for 1 minute, until small froth/foam forms. Add salt and cream of tartar and beat for 3 minutes, until a thicker foam forms.
In a separate small mixing bowl combine sugar and cornstarch. Once the eggs have become a thicker foam and while the mixer is still running, slowly stream the sugar mixture into the egg mixture. Turn the mixer to a speed 4 and continue mixing for 2 minutes.
Add lemon juice, lemon extract, and vanilla and whip on speed 6 for 13-14 minutes, until stiff peaks form.
Spoon ¼ cup - ⅓ cup of the meringue onto the prepared baking sheet. Repeat until all of the meringue is used. Press a spoon into the center of each meringue to form a bowl shape.
Place into the preheated oven and bake for 45 minutes. Once the time is up, turn off the oven and leave the oven door shut. Let the meringues cool in the closed oven for 1 hour and 30 minutes.
Once cooled, remove from the oven and place on a wire cooling rack.
Divide the curd evenly between the meringues, spooning the curd into the bowl shaped part of the meringue. Top each one with a lemon slice, dust with powdered sugar and serve.
Notes
Storage: Once fully cooled, store these lemon meringue cookies in an airtight container or bag for up to 4 days. If you're adding lemon curd as a topping, I recommend storing them in a container without overlapping them. Otherwise, the curd will stick to the bag's plastic. Don't refrigerate them because they'll absorb moisture and soften.