Melt the sugar, corn syrup, and salt together in a large, heavy-bottomed pot over medium heat. (A stainless steel pot works best. An enameled dutch oven pot will make the temperature harder to control and can lead to overcooking.)
Bring the syrup mixture to a boil and boil until it reaches 244F. Wash down the sides of the pot to remove any sugar crystals if needed, but there’s no need to stir once it’s boiling.
Meanwhile, add the egg whites to the bowl of a stand mixer fitted with the whisk attachment. Beat on medium-high speed for 2 minutes until starting to thicken/foam.
When the syrup mixture reaches 244F, slowly pour in ⅓ of the syrup while mixing on low speed. Pour the syrup in very slowly and carefully and be sure it’s being mixed the whole time.
Put the remaining syrup in the pot back on the stove and cook over medium heat until it reaches 264F.
With the mixer running on low, pour the rest of the syrup (slowly and carefully) into the egg whites.
Continue mixing on medium-high speed for 5 minutes, or until the mixture is stiff and starts to clump around the whisk. It won’t form completely stiff peaks but will thicken quite a bit.
With the mixer running on low, stir in the vanilla, and ½ the butter and flour. Mix in the rest of the butter/flour.
Divide in half and pour over the cooled caramel. Smooth into an even layer. Let cool completely.