Preheat oven to 350 degrees. Line a 9-inch square pan with parchment paper.
In a medium saucepan, melt the butter. Stir in the chocolate over low heat until the chocolate is melted and smooth.
Remove from the heat and stir in the sugar and vanilla until combined. Beat in the eggs one at a time. Add the flour and stir for 1 full minute until the batter becomes smooth and glossy, and pulls away a bit from the sides of the saucepan. Stir in the chopped nuts.
Spread the batter into the prepared pan and bake until the center feels almost set, about 30 minutes. Don’t over bake.
Let cool completely in the pan before lifting out the foil or parchment to remove the brownies. Cut into 12 pieces. Serve.