Preheat the oven to 375 °F. Butter a 9" square baking pan and line with parchment paper. You can just butter the pan but the parchment makes it much easier to remove the blondies from the pan.
In a large bowl, combine 6 oz unsalted butter (melted) with 12 oz brown sugar. Add 3 large eggs and 1 tablespoon vanilla extract. Mix to combine then stir vigorously for 30 seconds.
Add 7 ½ oz all purpose flour and ½ teaspoon table salt. Spread the batter into the pan.
Bake until the center springs back when lightly pressed, and a toothpick inserted in the center comes out with a few wet crumbs, about 25 minutes.
Cool to room temperature before removing from the pan and cutting into 16 squares.
Notes
If measuring the flour by volume use the “dip & sweep” method. That is, dip the measuring cup into the flour bin, overfill it, then sweep away the excess.