In a heavy saucepan, combine egg yolks, 1-1/4 cups sugar, lemon juice and egg whites. Bring to a boil over medium heat; cook and stir for 8-10 minutes or until mixture reaches 160° or is thick enough to coat a metal spoon. Remove from heat. Cool quickly by placing pan in a bowl of ice water; stir for 2 minutes. Stir in lemon zest. Transfer to a bowl, press plastic wrap onto surface of custard. Chill for 2-3 hours or until partially set.
In a large bowl, beat cream on medium speed until soft peaks form. Gradually beat in remaining sugar, 1 tablespoon at a time, on high until stiff peaks form. Gradually fold whipped cream into the cooled lemon mixture.
Arrange 24 ladyfingers around the edge of an ungreased 9-in. springform pan. Arrange 16 ladyfingers on bottom of pan. Spread with half the lemon mixture. Layer with remaining ladyfingers; top with remaining lemon mixture.
Cover and freeze overnight. Remove from the freezer 5 minutes before cutting. Remove sides of the pan. Garnish with lemon slices and mint.