Make the brownie batter as directed in the package and spread it on an 8x8-inch pan.
Add whole or chopped nuts on top.
Bake the brownies according to the package instructions.
Melt the chocolate in the microwave, then put it in a zip-top bag.
Snip off one end and drizzle over the brownies.
Sprinkle some flaky salt.
Let it set for at least an hour in the fridge before cutting.
Notes
Tips for best brownies:
Bring the eggs to room temperature and lightly beat them to incorporate them well without overmixing.
Grease and line the pan with parchment paper to prevent the brownies from sticking.
To avoid overbaking, check the brownies with a toothpick 2 minutes before they are done. Look for several separate crumbs for fudgy brownies and less for cakey brownies.
Cool the brownies before slicing. Refrigerating them improves their texture.