Start off by adding you blackberries to a medium saucepan together with your lemon juice and sugar.
8 oz blackberries, ¼ cup granulated sugar, 1 tablespoon lemon juice
Place this over a medium high heat and cook, stirring occasionally until the sugar has dissolved and the berries have broken down. Continue cooking until the mixture boils.
Once boiled, remove the pan from the heat and strain the mixture through a sieve to remove the pulp. Return the liquid to the saucepan, but keep off the heat.
2 egg yolks, 1 large egg
In a small bowl whisk together the eggs, then slowly add these to the saucepan (while off the heat), whisking the juices continuously.
Return the saucepan to a medium high heat, whisking regularly while it thickens. This normally takes about 15 minutes until the curd coats the back of a spoon.
Remove from the heat and add the butter, whisking through to melt.
2 tablespoon unsalted butter
Pour the curd into a heatproof bowl, jar or other container to cool completely. This will continue to thicken a bit as it cools, so you should expect it to still be pourable at this stage rather than spreadable.