I do not profess to understand all of the science behind it, but divinity has to be made on a dry precipitation free day. That can definitely be a challenge because this is traditionally a Christmas time treat and we live in an area that's prone to snow. I swear it's the truth though - if it's snowing, raining, foggy or you're dealing with really high humidity your divinity will not set up right.
Butter a cookie sheet or pizza pan and set aside
Add Sugar, Corn Syrup and Hot Water to a pot, attach your candy thermometer and bring to low boil over high heat.
While Sugar is cooking on stove top, add your Egg Whites, Salt and Cream of Tarter to a Stand Mixer and beat until eggs form a stiff peak. Set aside. (Make sure to stir your sugar mixture occasionally too)
Once Sugar Mixture reaches 248-250 degrees, remove from heat.
With mixer on low, very carefully drizzle in hot sugar mixture until it's all added. Add in Vanilla & Pecans once all of sugar mixture is added. Continue beating until mixture loses it's glossy shine and looks dull. You may need to alternate between your stand mixer and mixing by hand to give the mixer a chance to cool a bit. This can also all be done with a hand mixer, but if you do make sure it's not one you're attached to. Growing up, helping my grandma make this every year I can't even begin to tell you how many mixers she burned up. I've done my share of them too.
Once mixture is extremely stiff and dull in color scrape out onto your prepared pan to finish cooling. Once cool, cut into squares.