Preheat the oven to 350˚F.
Line a 9-inch square metal baking pan with parchment paper and spray lighting with nonstick cooking spray.
Heat the butter and chocolate in a bowl over a double boiler. Stir constantly with a rubber spatula until the butter and chocolate are fully melted and combined. Remove the bowl from the heat and cool to room temperature.
Whisk the eggs in a large mixing bowl. Add the granulated sugar, brown sugar, vanilla, and salt. Continue until they are all combined. Add the melted chocolate slowly and whisk until incorporated, about 1 minute.
Once the chocolate is incorporated, sift the flour into the bowl and gently stir with a rubber spatula until the flour is combined, about 1 minute.
Pour the brownie batter into the prepared pan. Bake for 45 to 50 minutes, rotating the pan once halfway through. Before removing the pan from the oven, gently shake it - if the middle of the brownies remains still, they are done.
Run a small metal spatula along the sides of the pan and allow the brownies to cool in the pan to room temperature, about 1 hour.
Using the parchment paper, lift the brownies from the baking pan. Using a sharp chef's knife, slice the brownies into squares the size you want. Serve immediately.