Begin by browning your butter. In a medium saucepan, melt the butter over medium heat. Stir with a heat proof spatula or spoon and cook over medium-high heat. It will bubble loudly, then right before it is finished the sound will quiet down. Stir and look to see if the milk solids have turned brown, and remove from the heat if so. It can burn very quickly and will keep browning if left on the heat or in the pot. Pour into a large, heat proof mixing bowl and let cool for 15-20 minutes.
While the butter cools down, preheat the oven to 350. Line an 8 x 8” metal baking pan with heavy duty aluminum foil (or butter the pan generously).
Stir the brown sugar and the salt into the butter. Next add the eggs and the vanilla (the butter can still be warm, but if it is too hot it will scramble the eggs in this step). Finally stir in the flour. The batter will be thick, so it is easier to use a spatula or wooden spoon for adding in the flour.
Scrape the batter into the prepared pan and smooth the top. Bake about 25 minutes. A cake tester will not come out clean, but the edges should pull away from the sides slightly and the top should feel somewhat firm to the touch and be shiny. Let cool completely before cutting.
Blondies will keep for a few days at room temperature, or for a month in the freezer.
I adore cinnamon, and often will add a teaspoon to the batter along with the flour. Other tasty add-ins could be walnuts or pecans, or reducing the vanilla to 1 teaspoon and adding in 1 tablespoon of dark rum.
My trick for getting the foil to line the pan easily is to flip the pan upside down, shape the foil around the bottom of the pan, then remove it, turn the pan over and fit the shaped foil down into the interior of the pan.