Line an 8 inch square pan with foil, leaving a 2-inch overhang on two sides.
Spray foil very lightly with vegetable spray.
In a medium-heavy bottomed saucepan, combine the sugar, cream and corn syrup. Stir before heating.
Cook over low heat, swirling the pan as necessary, until the sugar melts, about 10 minutes. Avoid getting any of the sugar on sides of the pan.
Increase heat to medium-high and bring to a boil. Boil, without stirring, until the mixture reaches 238 degrees Fahrenheit on candy thermometer, about 5 minutes.
Immediately transfer the mixture to the large bowl of a stand mixer and beat on medium-low for 1 minute, then increase speed to medium and beat until no longer shiny, 5-7 minutes.
Add the vanilla and salt, mix to combine, then fold in pecan pieces. Transfer the mixture to the prepared pan, pressing down to cover the bottom of the pan. Refrigerate until set, about 30 minutes.
In a double boiler, melt the Symphony chocolate bars and mini chocolate chips over simmering water. Spread the melted chocolate over the chilled candy mixture, then sprinkle with sea salt. Sprinkle with chopped cashews or macadamia nuts and drizzle with white chocolate, if using.
Refrigerate until set, at least 15 minutes.
Remove the candy in the foil onto a cutting board. Cut into 1-inch pieces. (The candy may bread into uneven pieces, that's okay, it will be delicious!)