Line a baking sheet with parchment paper to prevent sticking and for easy cleanup.
In a heavy-bottomed saucepan, combine unsalted butter, granulated sugar, light corn syrup, and salt. Place over medium heat and stir continuously until the butter is melted and sugar is dissolved.
Attach a candy thermometer to the saucepan and continue to cook, stirring occasionally, until the mixture reaches 300°F (hard crack stage), about 10-15 minutes.
Remove saucepan from heat and stir in three-quarters of the toasted macadamia nuts carefully as the mixture will be very hot.
Quickly pour the hot toffee mixture onto the prepared baking sheet and spread evenly with a silicone spatula.
Melt the chopped chocolate in a microwave-safe bowl in 30-second increments, stirring until smooth. Pour over the toffee and spread into an even layer.
Sprinkle the remaining toasted macadamia nuts over the melted chocolate and press gently to adhere.
Allow toffee to cool at room temperature for 1-2 hours until set. Break into pieces and enjoy.