Adjust oven rack to the middle position and preheat oven to 350 degrees. Line an 8-inch square baking pan with a foil sling and coat with nonstick spray. In a medium bowl, mix flour and salt together. Set aside.
In a saucepan over medium heat, combine the butter and brown sugar. Heat, stirring often, until the sugar has dissolved. Continue to cook about 1 minute longer; the mixture should bubble but not boil. Cool slightly, about 10 minutes.
To the cooled mixture, add egg, egg yolk, and vanilla and stir with a wooden spoon to combine. Add the flour mixture and stir until just combined.
Pour the batter evenly into the prepared pan and smooth the top. Bake until the center is springy to the touch and a toothpick inserted into the center comes out clean with a few crumbs attached, about 25 to 35 minutes. Do not over bake.
Cool completely in pan on a wire rack, about 2 hours. Remove from pan using foil sling and cut into 16 squares.
Yield: This recipe makes 16 (2-inch x 2-inch) bars.
Storage: Once cool, store covered at room temperature for up to 4 days.
Freezing: To freeze, wrap the cut-up Blondies tightly in plastic wrap, then transfer to an airtight container or freezer bag. Freeze for up to three months. Thaw at room temperature.
Don't over-bake. Above all, a homemade Blondie should be soft and chewy. They'll continue to firm up as they cool in the baking pan.
White chocolate: For bites of extra sweetness, stir 1/2 cup white chocolate chips into the batter before baking.