Preheat the oven to 325 degrees F.
Melt the butter in a saucepan over medium-low heat.
Whisk in the sugar, cocoa powder, vanilla extract, and salt until smooth.
Let the chocolate mixture cool slightly, about 5 minutes off the heat.
Whisk in the eggs until fully incorporated.
Add the flour and whisk only until combined (40 to 50 strokes is perfect).
Pour the brownie batter into a parchment paper-lined or well-greased 8x8-inch baking dish.
Bake for 30 minutes, or until edges are set and the middle doesn't jiggle when you shake the pan.
For the cleanest squares, cool before cutting.