Preheat oven to 350°F. Line an 8-inch square metal baking pan with foil, draping foil over edges. Mist with cooking spray.
In a small saucepan, melt butter over medium heat. Remove pan from heat and stir in chocolate until melted and smooth. Let cool.
In a large bowl, whisk together sugars, eggs, vanilla and salt until smooth and pale, about 30 seconds. Stir in cooled chocolate mixture and buttermilk. Stir in flour until just combined, then pour batter into baking pan.
Bake until a toothpick inserted into brownies comes out clean, 40 to 45 minutes. Transfer pan to a rack and let cool completely. Lift brownies out of pan, remove foil and cut into 12 pieces. Wrap brownies in plastic wrap and store at room temperature for up to 5 days.
For these six variations, bake until a toothpick inserted into brownies comes out clean, 55 to 60 minutes.
Coconut-Macadamia: Substitute 1/2 cup coconut oil for butter and 1/2 cup coconut milk for buttermilk. Stir in 1/2 cup toasted, chopped macadamia nuts with flour. Sprinkle 1/4 cup flaked coconut over batter before baking.
Peanut Butter and Pretzels: Substitute 1/2 cup smooth peanut butter for buttermilk and stir in 1/2 cup Reese's Pieces with flour. Sprinkle 1/2 cup crushed pretzels over batter before baking.
Coffee Bean: Stir in 2 Tbsp. instant espresso powder with flour.
Malted Toffee: Stir in 1 cup toffee bits and 1/4 cup malted milk powder with flour. Sprinkle 1/4 cup toffee bits over batter before baking.
Oreo Cheesecake Swirl: Stir in 3/4 cup quartered Oreos (about 6 cookies) with flour. In a small bowl, stir 4 oz. room-temperature cream cheese, 2 Tbsp. sugar, 1/4 tsp. vanilla extract and 1/4 tsp. kosher salt until smooth. Scrape into a small ziplock bag. Snip off 1 corner of bag with scissors, then squeeze cream cheese mixture in a swirled pattern over the batter in pan.
Honey-Nut: Substitute 1/2 cup honey for the brown sugar, and use 1 full cup sugar. Stir in 1/2 cup chopped, roasted mixed nuts with flour.