Heat oven to 350° F. Spray an 8-inch square baking pan with the cooking spray. Line the pan with 2 crisscrossed pieces of parchment paper, leaving an overhang on all sides; spray the parchment with the cooking 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. Remove from heat and mix in the sugar and vanilla. One at a time, mix in the eggs; mix in the flour mixture until just combined (do not overmix).
Spread the batter in the prepared pan and bake until a toothpick inserted in the center comes out clean, 28 to 32 minutes. Cool completely in the pan.
Holding both sides of the paper overhang, lift the cake 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.
Makes 16 brownies