Combine the chocolate, sugar, cream, corn syrup, and butter in a saucepan over low heat.
Bring to a boil, stirring frequently, and simmer until the candy thermometer reads 245 F (118 C).
Remove from heat and cool in a water bath until the thermometer reads 125 F (50 C).

Stir in the nuts, vanilla, and orange zest, then beat with an electric mixer until the mixture is creamy and thickened (about 5 minutes).
Pour the mixture into an 8″ (20 cm) square mold lined with parchment paper.
Let cool completely before cutting into squares.