To begin, get all of your ingredients ready. To make your fruit purée, take about 1 cup of fresh fruit of your choice, removing any skins as needed, and blend on high in a blender until smooth and creamy. See notes below on how to make a fruit purée using a water-dense fruit.
Also, prepare your butter by cutting it into cubes. Set aside.
To a saucepot, add in the 4 egg yolks and one whole egg. Whisk together gently (you don't want to introduce too much air), until the mixture is a little more homogenous. Do not put the pot on heat yet.
Add in the sugar, lemon juice, fruit purée, vanilla extract, and salt to the saucepot. Whisk these with the eggs until combined.
Now, put the saucepot on the stove and bring to low heat, whisking CONSTANTLY. Whisking constantly is the name of the game here. Continue to whisk for anywhere between 5-12 minutes, or until fruit curd is thickened. At the beginning, the curd will be runny and liquid-y. When it's done, it will be thicker, almost like the consistency of pudding, but still pourable.
To test when the fruit curd is ready, dip a spoon into it. If the curd coats the back of the spoon when you pull it out, in a nice even, glossy layer, then it's done. If you have not reached this stage yet, continue whisking on low heat.
When done, remove the curd from the heat. Add in the cubed butter to the hot curd, and stir until all of the butter is melted.
Now, pour your curd through a mesh sieve into a bowl. You MUST strain it to remove any fruit pieces or egg pieces that it may contain, otherwise your curd will not be smooth.
Let the curd cool slightly (you don't want to put it in the refrigerator piping hot), then cover it with cling wrap. Put the curd in the refrigerator, and let chill for at least 2 hours.
When ready, serve and enjoy!