Step 1

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.

Step 2

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.

Step 3

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.

Step 4

Switch to the whisk attachment. Mix on high for 7-8 minutes, or until the frosting is smooth and creamy.