Step 1

Heat the oven to 375ºF with a rack in the center. Spray an 8×8-inch metal square cake pan with nonstick cooking spray, then line with parchment (see steps and diagram above), leaving an overhang on all four sides.

Step 2

Place the butter in a small saucepan or skillet over medium heat. Melt the butter until it starts to foam and sputter, 2-4 minutes. Whisking often, continue to cook the butter until it starts to quiet down, smells nutty, and turns deeply golden brown, another 3-5 minutes. As you whisk, try to lift any browning milk solids that are stuck to the pan. Remove from the heat and immediately pour into a large heatsafe bowl placed over a digital scale, making sure to scrape in as many browned bits stuck to the pan as possible. You should have about 113 grams of butter remaining.

Step 3

Immediately whisk the cocoa and vanilla into the warm brown butter until smooth. Whisk the sugar and salt into the brown butter mixture until combined. Beat in the eggs one at a time, whisking vigorously to combine, about 1 minute (after the first egg the mixture will look lumpy and separated but after beating in the second it will form into a smooth, glossy mixture). Fold in the starter discard until combined, then fold in 59 grams (2 ounces, about 6 tablespoons) of the chopped chocolate.

Step 4

Pour the batter into the prepared pan and smooth as needed to make an even layer. Sprinkle over the remaining chopped chocolate. Bake until a tester inserted into the center comes out with some moist (not totally gooey) crumbs, 29 to 32 minutes. Immediately sprinkle the surface with flaky salt, then let cool for at least 35 minutes, but ideally 1 hour—these are fudgy! (If you can, let the pan cool at room temperature for at least 15 minutes so it’s not searing hot, transfer the pan to the refrigerator and chill for at least another 15 minutes, or up to overnight.)

Step 5

Lift the brownies out of the pan by the parchment and place on a cutting board. Cut into pieces (I like cutting the batch into 9 large, 12 medium, or 16 bite-sized pieces). Brownies will keep in an airtight container at room temperature for up to 5 days, in the fridge for up to 10 days (I love cold brownies, this is my preference), or in the freezer for up to 4 months.