In a medium saucepan, mix together sugar and cornstarch.

Add in milk, yogurt, lemon juice, zest, extract and yolks.

Whisk till combined.

Turn heat on to medium. Cook till mixture starts to bubble and thicken.

Lower heat to simmer and let cook for 2 minutes.

Take off heat and stir in butter.

Let cool, cover with plastic wrap (pushing down to touch surface of pudding), place in fridge to set. Should be ready in 3 hours.