Blondies—the fair-haired cousin of Brownies—likely originated during World War II due to the need to bake without rationed ingredients like chocolate and white sugar. It seems odd, I know, because packed as this treat is with butter, brown sugar, and eggs, it just doesn’t seem like a recipe in which you are doing without something you love.
Place a rack in the center of the oven, and preheat the oven to 350°F. Lightly grease and flour a 13" × 9" baking pan and set it aside.
Place the butter in a large saucepan over medium heat and stir until melted. Add the brown sugar and stir with a wooden spoon until the sugar dissolves and the mixture thickens, about 3 minutes. Turn off the heat. Add the granulated sugar and stir until well combined, 1 minute. Let cool slightly.
Whisk together the flour, baking powder and salt in a medium size bowl. Add a third of the flour mixture to the saucepan and stir to combine and bring down the temperature of the butter and sugar mixture, 30 seconds. Add half of the beaten eggs and the vanilla. Stir to combine, 30 seconds. Add another third of flour mixture, stir to combine, then add the rest of the eggs, then the last of the flour, and stir until smooth. Turn the batter into the prepared pan and sprinkle the top with pecans, if desired. Place the pan in the oven.
Bake the blondies until nut brown around the edges and just firm in the center, 20 to 25 minutes. You do not want to overbake.
Remove the pan from the oven, and place it on a wire rack to cool. Score the blondies into pieces with a sharp knife. When completely cool, slice into pieces and serve. These blondies keep covered at room temperature for up to 4 days and in the freezer for up to 4 months.