Preheat your oven to 350°F (175°C). If using parchment paper, line your baking pan with it, leaving some overhang.
In a medium saucepan over low heat, combine the butter and semisweet chocolate. Stir continuously until melted and smooth. Remove from heat and allow to cool slightly.
Whisk in the sugar and vanilla extract until well combined.
Add the eggs one at a time, whisking well after each addition until the batter is glossy and smooth.
Gently fold in the flour until just combined. Avoid overmixing.
Pour the batter into the prepared baking pan and spread evenly. Bake for about 20-25 minutes or until a toothpick comes out with a few moist crumbs.
Allow the brownies to cool in the pan for about 10 minutes before lifting them out using the parchment paper. Cool completely on a wire rack before slicing into squares.
