Check the weather first: choose a dry, low-humidity day. Line two baking sheets with parchment or waxed paper and set out two spoons for dropping.

In a heavy 2-quart saucepan, stir together the sugar, corn syrup and water over medium heat until the sugar dissolves and the mixture comes to a boil.

Stop stirring. Clip on a candy thermometer and cook the syrup, without stirring, until it reaches 260°F (127°C) — the hard-ball stage. This takes about 15–20 minutes. (Without a thermometer: a drop of syrup in cold water should form a hard ball that stays firm but still gives a little when squeezed.)

When the syrup is close to temperature, beat the egg whites in a large, clean bowl with a mixer until stiff peaks form — they should stand up straight and hold.

With the mixer running on medium-high, pour the hot syrup over the whites in a very slow, thin stream. Take your time — a full minute or two. Scrape the pan's last drizzle in carefully; the syrup is hot enough to burn, so keep hands and helpers clear.

Add the vanilla and keep beating on high until the candy loses its gloss, thickens, and holds its shape when the beater is lifted — 4–6 minutes more. It should look like stiff, dull marshmallow fluff. Stir in the chopped pecans by hand.

Working quickly (it sets fast now), drop the candy by spoonfuls onto the prepared sheets, swirling the top of each piece with the back of the spoon. Press a pecan half on top if you like.

Let the pieces stand at room temperature 1–2 hours, until firm and dry to the touch, before packing into a tin.