Preheat the oven to 350º. Line an 8 x 8-inch pan with nonstick foil or regular foil (spray with cooking spray). Set aside.
Melt chocolate and butter in a microwave-safe bowl, stopping and stirring at 30-second increments till melted and smooth.
Stir in the sugar and vanilla.
Beat in the eggs one at a time. Add the flour and whisk vigorously for a full minute. The batter should look smooth and glossy and sugar granules should be incorporated.
Spread batter into pan and bake for about 40 minutes until the brownies look almost set. Do not overbake.
Let cool completely in the pan before lifting out the foil or parchment to remove the brownies and slice into squares.
