Lemon Filling or Spread
- Fill a medium saucepan with 1-inch water and place over medium-high heat until it simmers.
- In a bowl large enough to hold all ingredients whisk zest, lemon juice, and sugar together.
- Add eggs and vigorously whisk until no strings of eggs are visible.
- Place bowl over simmering water and turn heat down to a very low simmer. Whisk constantly until the mixture thickens, about 9-12 minutes.
- Remove from heat and allow to sit for 3-4 minutes.
- Add butter and whisk into the mixture. Cover and keep refrigerated.