Preheat oven to 350 degrees Fahrenheit. Line a 9x13 inch pan with greased parchment paper and set aside. OR preheat oven to 325 degrees Fahrenheit and line a 9x9 inch pan with greased parchment paper and set aside.
In a medium saucepan, over medium heat, melt your butter and chocolates (both semi-sweet chocolate chips and unsweetened chocolate) together. Keep stirring until they are completely melted and incorporated together. Add in both sugars and stir until combined.
Remove saucepan from heat and let cool slightly. Your mixture should be warm, but not hot (you don’t want the eggs to scramble and cook when you add them in). Add in 1 egg at a time and really whisk the mixture so the egg is fully incorporated and glossy before adding in the next egg. Repeat until all the eggs are mixed in.
Add in your salt, vanilla, and flour (make sure your flour is spooned, leveled, and measured correctly—too much flour will make these cakey not fudgy). Mix until the flour is just barely incorporated. Do not over mix.
Pour batter into your greased and lined baking dish. And if using a 9x13 inch pan bake for 30-40 minutes (until a shiny crust has formed and the brownie batter is moderately firm. OR if using a 9x9 inch pan, bake for 35-40 minutes.
Cool completely before eating (trust me, they’re better). I like to cover the pan in plastic wrap and let them sit until the next day to maximize the fudgy texture!