Combine egg yolks, 1/4 cup granulated sugar, 1/4 cup reserved lemon juice, and flour in the heatproof bowl of an electric mixer fitted with the whisk attachment. Beat mixture on medium speed until pale yellow, about 3 minutes. Place bowl over a pan of simmering water; whisk constantly until very thick, about 8 minutes. Remove bowl from heat, and return to mixer. Beat on medium speed until cool, scraping down sides several times, about 10 minutes. Transfer to a medium bowl, and set aside.