How to Make It
Heat oven to 350°F. Coat an 8-inch square baking pan with nonstick spray. Line the pan with 2 crisscrossed pieces of parchment paper, leaving an overhang on all sides; coat the parchment with nonstick spray.
In a medium bowl, whisk together the flour, cocoa powder, and salt.
In a medium saucepan, melt the butter and chocolate, stirring often, over medium-low heat, until melted. Remove from heat and stir in sugar and vanilla. One at a time, mix in the eggs and egg yolk; add the flour mixture and mix until just combined (do not overmix).
Spread the batter in the prepared pan and bake until the center is firm and set, 28 to 32 minutes. Cool completely in the pan.
Holding both sides of the paper overhang, lift the brownies out of the pan, transfer to a cutting board, and cut into 16 squares. Store the brownies in an airtight container at room temperature for up to 5 days.