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Preheat the oven to 350 degrees F. Lightly grease a 9x13 baking pan and line the bottom and sides with parchment paper. I use a single sheet of parchment paper and cut slits in the paper at the corners to fit the sheet into the pan. This method is preferable because, when the cookies cool in the pan, the entire batch can be lifted out onto a cutting board, making them much easier to cut cleanly and evenly.
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Sift together the flour, baking powder, salt and baking soda. Set aside.
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Melt the butter in a small saucepan and remove from heat.
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Add the brown sugar, eggs and vanilla, stirring until the brown sugar has dissolved. Pinch the mixture between your fingers to make sure that you cannot feel any sugar granules.
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Mix in the dry ingredients. Spread the batter into the prepared 9x13 baking pan.
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Bake for about 30 - 35 minutes at 350 degrees F (325 F for glass bake ware) or until a wooden toothpick inserted into the center comes out clean. Cool for 20 minutes before adding the top layer. The blondies should still be warm but not cooled completely when the top is added. If the blondies are too cool when the topping is added the top will not adhere to the bottom layer as well.