In a medium saucepan, combine the blueberries, granulated sugar, fresh lemon juice, lemon zest, and salt. Cook over medium heat, stirring occasionally, until the blueberries burst and the mixture begins to simmer, about 5-7 minutes. You'll see the liquid turn a beautiful deep purple.

Carefully pour the hot blueberry mixture through a fine-mesh sieve into a clean, heatproof bowl. Use the back of a spoon or a flexible spatula to press firmly on the solids in the sieve, extracting as much liquid and pulp as possible. Discard the blueberry skins and remaining solids in the sieve. You should have a smooth, vibrant blueberry liquid.

In a separate medium bowl, whisk together the egg yolks and the whole egg until they are well combined and smooth. This ensures no stray whites and good emulsification.

Slowly drizzle about half of the warm blueberry liquid into the whisked egg mixture, whisking constantly and vigorously. This step, called tempering, prevents the eggs from scrambling when they hit the hot liquid.

Pour the tempered egg mixture back into the saucepan with the remaining blueberry liquid. Add the cold butter pieces. Cook over medium-low heat, stirring constantly with a heatproof spatula or whisk. Continue stirring until the mixture thickens significantly and coats the back of a spoon, about 8-12 minutes. Do not allow it to boil, or the eggs may curdle. The ideal temperature for setting is around 170-175°F (77-79°C).

Remove the saucepan from the heat and stir in the vanilla extract. Pour the finished blueberry curd into a clean jar or other heatproof container. To prevent a skin from forming, press a piece of plastic wrap directly onto the surface of the curd. Chill in the refrigerator for at least 4 hours, or preferably overnight, until it's thoroughly cold and set.