Preheat your oven to 220 degrees Fahrenheit. Afterward, take your baking sheets. And line them with parchment paper. Then, set them aside. Make sure that the mixing bowl is dry and clean. Also, the egg whites should not have any egg yolks.

Next, brush the inside of your mixing bowl and the whisk attachment with the lemon slice.

After that, place the egg whites inside the mixing bowl. Then, whisk the egg whites and vanilla using an electric mixer. Make sure to beat at medium-high speed. Do so till the egg white mixture forms soft peaks.

Then, slowly add in your sugar. Remember, add one tablespoon at a time. Do this step while the mixer is still running. Also, beat properly after each addition until all the sugar incorporates well.

When you add all of your sugar, beat the mixture until the sugar dissolves. In addition to that, the mixture becomes glossy and stiff for about four minutes. You must dissolve the sugar not to become gritty when you rub the mixture between your fingers. Besides that, avoid overmixing.

After that, scoop the meringue using a rubber spatula. And place it into a large pastry bag with a large star tip. Then, pipe a 1.5-inch cookie into your prepared baking sheets. And leave an inch gap between each of your cookies. Remember, the meringue cookies will increase in size as they dry out.

Next, bake your cookies inside the preheated oven for about sixty minutes. After the time intended, please turn off your oven. And leave the cookies inside the oven. Refrain from opening the oven door until they cool. Doing this will allow them to dry completely.

Please note that cookies should be crispy outside. So with that, lift your cookies from the baking sheet without sticking. And it should sound hollow when tapped at the bottom. When your meringue cookies cooled completely, transfer them to an airtight container. Likewise, you can serve them immediately.