Preheat the oven to 350°F and lightly grease a rectangle-shaped baking pan. (A 13x9 inch pan will do just fine.)
To make the crust, mix one and one-quarter cups of Bisquick and five tablespoons of brown sugar until fully incorporated. (I prefer to use a food processor)
Add eight tablespoons of cold butter to the Bisquick mixture, pulsing until the butter resembles tiny pea-sized pieces, and the dough appears crumbly.
Spread the dough evenly into the prepared baking pan using your hands and press down to create a packed surface.
Place the pan into the center of the oven and bake for twenty minutes or until golden.
To make the filling, beat four large eggs in a large bowl using an electric mixer until well blended. Add one-half teaspoon of vanilla extract and three-quarters of a cup of powdered sugar and mix well. Add three-quarters of a cup of lemon juice and the remaining three-quarters of a cup of powdered sugar. Continue mixing until the powdered sugar is dissolved and add two teaspoons of lemon zest.
Remove the crust from the oven and lower the oven temperature to 300°F.
Pour the lemon filling into the hot crust and bake for an additional thirty minutes.
Cool the lemon bars completely before serving.
Sprinkle with powdered sugar and enjoy.