1. Preheat the oven to 300°F or 150°C.

2. Line a baking sheet with parchment paper.

3. Whisk the egg yolks and sugar together in a medium bowl until they become pale and fluffy, then set this mixture aside.

4. Whisk together the softened butter and lemon zest in a separate bowl until they are well combined.

5. Add the lemon juice to the butter mixture and whisk it until smooth.

6. Gradually pour the lemon-butter mixture into the egg yolk mixture while continuously whisking.

7. Pour the mixture into a saucepan and cook it over low heat, stirring constantly, until the mixture has thickened and coats the back of a spoon. This process should take about 10-15 minutes.

8. Take the saucepan off the heat and let it cool slightly before transferring the mixture to an airtight container.

9. Refrigerate until the mixture is chilled and set.

10. Once set, you can use this mixture as a spread on toast or scones or as a cake filling.