In a medium saucepan, whisk eggs, egg yolks and sugar together until smooth. Whisk the corn starch together with some of the lemon juice to create a slurry.
Whisk the lemon juice and the slurry into the egg mixture. Add the sage leaves.
Continue to whisk over medium heat until thick, 10-15 minutes. It should be the consistency of pudding. Make sure the whisk hits the bottom of the pot to keep the bottom from burning. This is less likely to happen if you use a heavy bottom saucepan.
Remove from the heat and slowly whisk in pieces of the butter.
Move the curd to a bowl, allow it to cool for several minutes and then press plastic wrap against the top of the curd to prevent a skin from forming. Allow to chill in the refrigerator with the sage leaves. It will continue to thicken as it cools.
After 2 or more hours, remove the sage leaves either by hand or by using a fine mesh strainer. Make sure to squeeze any curd from the leaves back into the bowl (that curd will have the most sage flavor). Once the leaves are removed, stir the curd to evenly disperse the sage flavor.
Store curd in the refrigerator with plastic wrap pushed directly against the curd to prevent a skin from forming.
Serve on biscuits, scones, in parfaits or with a spoon.