Line a baking sheet with parchment paper and set aside.
Double-check that your mixer bowl is extra clean and not greasy, as that will cause the egg whites to not whip up properly. In a bowl of an electric mixer fitted with the whisk attachment, whisk egg whites and salt on low-medium speed until foamy and soft peaks start to form, approximately 40-50 seconds. Then, while mixer is running, add the sugar, one tablespoon at a time, and continue whisking on medium-high speed until stiff peaks form, meringue is glossy and completely smooth but not dry, and the sugar is fully dissolved (this part of adding the sugar and continues beating may take 5-10 minutes.). Beat in vanilla extract. Stop the mixer and add the powdered sugar. Fold gently by hand until combined and smooth. Fold in food colouring, if using.
Immediately spoon mounds of meringue onto the prepared baking sheet, about 2.5-inch in diameter (for piping or “kisses” shape, refer to above post). These cookies don’t spread much, so you can place them quite close to each other. Place in the lower part of the oven and bake for 2-3 hours, or until the meringues are dry, their centre isn’t sticky, and they are easily lifted from the parchment paper with their bases intact. Turn off the oven and, without removing, let meringues cool completely for at least one hour or overnight.
Meringue cookies will keep for 2 weeks in an airtight container at room temperature.