Place the blueberries, lemon juice and lemon zest in a saucepan and heat over medium heat until simmering.
Cook for 5-7 minutes, until blueberries have softened and start popping.
Remove from heat and strain through a mesh strainer or sieve into a bowl, pressing the mixture firmly to the sides to extract all the juice and pulp from the blueberries. (Don't forget to scrape the bottom of the strainer into the bowl as well).
Add the blueberry mixture and sugar back to the saucepan and heat over medium low heat until sugar has fully dissolved and is no longer grainy.
Remove from heat and let cool slightly.
Beat the egg yolks in a bowl.
Temper the egg yolks by adding a couple tablespoons of the blueberry mixture to the egg yolks while whisking quickly to avoid cooking the yolks.
Add a couple more tablespoons and keep doing the same. Once the egg yolk mixture has been slowly warmed, whisk it into the saucepan with the rest of the blueberry mixture.
Heat the mixture over medium heat until it just starts to simmer.
Whisk in the butter, one tablespoon at a time, letting it melt before adding the next tablespoon.
Remove from heat and pour into a heatproof bowl. Cover with plastic wrap and let it cool at room temperature for 30 minutes.
Pour into a glass jar or covered container and store in the fridge for up to 2 weeks.