Combine 1 cup sugar, lemon juice, 8 egg yolks and lemon zest in a small saucepan. Whisk to combine.
Over medium heat, stir constantly with a wooden spoon.
Cook until mixture is thick enough to coat back of wooden spoon, about 7 minutes.
Place lemon mixture in fridge and chill until cool, stirring occasionally.
In a separate bowl, mix graham cracker crumbs and butter until well combined. Divide graham cracker mixture between 8 serving dishes. Press into bottoms and set aside.
Using electric mixer, beat heavy cream in medium bowl until medium peaks form. Do not add any sugar.
Gently fold whipped cream into lemon mixture until well combined.
Spread mousse over graham cracker crumbs and garnish with extra lemon zest.
Notes
Store leftovers in the fridge in an airtight container!