Set the blueberries in a medium sized pot over medium low heat. Cook as the berries burst and break down, until you have mostly a sauce - this will take about 20 minutes or so. Let cool briefly then transfer to a food processor and puree. Press the puree through a fine mesh sieve back into the (now cool) pot, do your best to squeeze out all the puree and leave only the blueberry skins. Stir in the sugar.

In a bowl, beat the eggs with the salt best you can (we want to break up the white bits as much as possible). Add the eggs to the pot and whisk the mixture together. Turn the heat onto medium low and cook until the mixture thickens; it will thicken first on the bottom so it needs to be stirred often to cook evenly. Keep a thermometer handy, it’s done when the temperature reaches 170 F.

Slice the butter and set it in a bowl, then place a fine mesh sieve and pour the cooked curd through it (we are sieving again to remove any egg bits). Once it’s all through, stir in the butter until the butter has melted and incorporated into the curd. Let cool to room temperature then transfer to a container to store in the fridge.