- One half cup (1 stick, 4 ounces) unsalted butter
- 2 cups (15 ounces) packed dark brown sugar
- 3 large eggs
- 1 tablespoon vinegar, preferably apple cider vinegar
- 1 teaspoon vanilla extract
- 1/4 teaspoon butter-rum, butterscotch or butter-pecan flavor (optional but delicious)
- 1 teaspoon baking powder
- 1 teaspoon salt
- 1 1/2 cups (6 ounces) traditional whole wheat flour
- 1 1/2 (5 5/8 ounces) chopped pecans or walnuts
- Preheat the oven to 350° F. Lightly grease a 9×13-inch pan. Melt the butter in a microwave-safe bowl or in a saucepan set over low heat. Remove from the heat and add the sugar, mixing until well blended. Transfer the mixture to a mixing bowl and let it cool to lukewarm.
- Add the eggs one at a time, beating well after each addition. Beat in the vinegar, vanilla, flavor (if using), baking powder and salt. Stir in the flour and nuts. Spread the batter in the prepared pan.
- Bake the blondies until the top looks shiny, 26 to 28 minutes. Don't overbake; bake just until the edges start to pull away from the sides of the pan and a sharp knife tip poked into the very center reveals sticky crumbs but not wet batter. Remove from the oven and run a knife all around the edge of the pan; this will help keep the blondies flat as they cool, by allowing the edges to settle in the pan along with the middle. Cool completely before cutting. If you prefer squares with a very smooth texture, wait 24 hours before cutting; this waiting period gives the bran in the whole wheat time to soften up.