Preheat the oven to 350 degrees F. Line an 8 inch square metal baking pan with foil and spray with baking spray or line with parchment paper.
Melt the butter in a saucepan set over medium heat. Add dark chocolate and milk chocolate Kisses; reduce heat to low and stir until melted and smooth. Remove from heat and stir in the sugar, syrup, salt and vanilla. Gradually stir in the lightly beaten eggs, followed by the flour. When flour is incorporated, stir in the macadamia nuts (if using).
Pour the mixture into the prepared pan and bake for about 25 minutes or until the brownies appear set. Remove from the oven and immediately scatter the chopped Hershey’s Kisses over the top. Return to the oven for exactly one minute.
Let the brownies cool to room temperature, then chill thoroughly.
Grasp foil, lift from pan and cut into squares while cold.
Make sure to chill the brownies thoroughly before cutting. They taste even better on Day 2.