1. Preheat oven to 350 degrees. Butter a 13-by-9-inch baking sheet. Line baking sheet with parchment paper, leaving a 1-inch overhang on the two long sides. Set aside. Place butter and chocolate in a large heatproof bowl set over (but not touching) simmering water; stir frequently until chocolate and butter are melted, about 7 minutes. Remove bowl from heat; let cool to room temperature, 10 to 15 minutes.

2. Stir sugar into cooled chocolate mixture until combined. Whisk in eggs one at a time, whisking until smooth after each addition. Whisk in vanilla. Gently fold in flour and salt.

3. Pour batter into prepared pan, and smooth top with an offset spatula; scatter green sprinkles over top. Bake until a cake tester inserted in the center comes out with only a few moist crumbs attached, 40 to 45 minutes. Transfer pan to a wire rack to cool completely.

4. Run a knife or offset spatula around the edges of the pan. Using parchment paper, lift brownies out of pan and onto the rack. Transfer to a cutting board; cut into 2-inch squares. Brownies can be kept in an airtight container at room temperature for up to 3 days.