1. In a small bowl, add lemon zest and sugar. Using your hands rub the zest into the sugars. This helps release the lemon oils and gives the utmost lemon flavor.

2. Add the lemon sugar and yolks to a small saucepan. Whisk to combine.

3. Whisk in the lemon juice, gradually until combined.

4. Over medium-low heat, heat the mixture for roughly 10 minutes, whisking constantly until thickened. The mixture should coat the back of a wooden spoon. The texture should be like pourable pudding.

5. remove from the heat and stir in the cold, cubed butter until completely melted and combined.

6. This step is optional, but you can strain the mixture through a fine mesh sieve to give a really nice, smooth consistency.

7. Pour the mixture into a resealable jar. Place in fridge to cool for 4 hours. The mixture should be like spreadable pudding once completely chilled.