Preheat the oven to 200°F (95°C). Line a baking tray with parchment paper. Make sure your mixing bowl and whisk are completely clean, dry, and grease-free.

Place 3 large egg whites in a mixing bowl. Add cream of tartar if using. Beat with an electric mixer until the egg whites become foamy and form soft peaks.

Gradually add the caster sugar, one tablespoon at a time, while continuing to beat. Mix until the sugar dissolves and the meringue becomes thick, smooth, and glossy.

Continue beating until stiff peaks form and the mixture holds its shape when the whisk is lifted. Add vanilla extract if using and mix briefly. Avoid overbeating.

Transfer the meringue mixture into a piping bag or use a spoon to form approximately 24 small meringue cookies. Arrange them on the prepared baking tray, leaving a little space between each cookie.

Bake for approximately 90 minutes at 200°F (95°C), or until the cookies are dry and release easily from the parchment. Ensure the centers are thoroughly cooked, reaching at least 165°F (74°C) when checked with a suitable food thermometer.

Turn off the oven and leave the meringue cookies inside for approximately 1 hour to cool and dry gradually. Once completely cooled, transfer them to an airtight container.