• Preheat oven to 350°F. Line a 9×9-inch baking dish with parchment paper.
• In a large bowl, combine the melted butter and brown sugar. Stir well.
• Add the eggs one at a time, stirring well after each addition.
• Stir in the lemon juice and lemon zest.
• In a separate, medium-sized bowl, whisk together the flour, cornstarch, baking powder, and salt.
• Gradually stir the flour mixture into the butter and egg mixture until completely combined.
• Spread the batter evenly into the prepared pan. Bake for 30-35 minutes, or until the edges are turning slightly golden brown. (You don't want to overbake these brownies!)
• Allow brownies to cool completely before frosting.

Lemon Frosting

• While the brownies are cooling, make the lemon frosting. In a medium saucepan over medium heat, whisk together the lemon juice and flour. Cook, whisking constantly, for 1-2 minutes, or until boiling. Boil for one minute, still whisking constantly. Remove from heat and bring to room temperature.
• In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar on medium speed until light and fluffy (2-3 minutes). Stop the mixer and use a rubber spatula to scrape down the sides of the bowl.
• Add the room-temperature lemon mixture to the bowl of the stand mixer. Beat on medium-high speed until the frosting is light and fluffy (2-3 minutes), pausing to scrape down the sides of the bowl as needed.
• Switch to the whisk attachment. Mix on high for 7-8 minutes, or until the frosting is smooth and creamy.
• Spread the frosting evenly over the cooled lemon brownies. Slice and serve.