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1
Preheat oven to 350 F. Line an 8x8 inch metal pan with foil and allow two ends of the foil to hang over the dish (you'll use the foil as a handle to remove the blondies when done). Spray foil with non-stick cooking spray, and set aside.
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2
Sift the flour, cinnamon, and salt into a large bowl, and set aside.
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3
Place the butter, dark brown sugar, and white sugar into a small saucepan. Cook on medium low heat, stirring frequently until the butter and sugars melt and start to bubble (do not let the mixture boil.) Remove pan from heat and set aside to cool for 15 minutes, stirring occasionally.
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4
Transfer cooled butter mixture to a medium bowl, and whisk in whole egg, egg yolk, and vanilla extract. Pour the wet ingredients over the flour mixture, and stir until just combined, and no flour streaks remain. Transfer batter to the prepared pan, spreading it to cover the entire pan. Bake for 20-25 minutes, until golden around the edges and no longer jiggly.
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5
Place pan on a wire rack, and cool completely before removing blondies from the pan.
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6
To make the buttercream, place the butter in a large bowl, and beat on low speed using your mixer. Gradually add the powdered sugar, and beat on medium speed until incorporated. Add the vanilla, and continue to beat on medium speed until the frosting is light and fluffy.
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7
Remove blondies from the pan using the foil 'handles'. Spread frosting over the top of the blondies and cut into 16 bars.
