salted caramel filling

2½ cups sugar
⅔ cup water
1 tablespoon light corn syrup
¾ cup heavy cream
2½ teaspoons sea salt, preferably fleur de sel

Heat sugar with the water and corn syrup in a heavy saucepan over high heat, stirring occasionally, until syrup is clear; clip a candy thermometer to side of pan. Stop stirring, and cook until syrup comes to a boil, washing down sides of pan with a wet pastry brush as needed. Boil, gently swirling pan occasionally, until mixture is caramelized and just reaches 360ºF. Remove from heat; slowly pour in cream (caution: mixutre will spatter and bubble up) and stir with a wooden spoon until smooth. Stir in sea salt. Use immediately. If at any time caramel begins to harden, reheat gently until pourable.