Step 1 In a saucepan, combine the lemon juice, butter and one-half cup sugar over medium heat.
Step 2 Whisk together the remaining sugar and the cornstarch. Add the eggs and whisk until smooth.
Step 3When the lemon juice mixture comes to a boil, remove it from the heat and slowly pour it into the egg mixture, whisking constantly, until fully incorporated.
Step 4Return the mixture to the saucepan and whisk vigorously over medium heat until thickened, about 4 minutes.
Step 5Strain the curd through a fine mesh strainer into another bowl and cool by placing it in a larger bowl filled with ice cubes and water. Set aside. (Makes 2 1/2 cups.)