Use a heat proof bowl such as glass or metal, that can sit on a medium or large cooking pot, without touching the bottom of the pan. (Known as a double boiler or Bain-marie). Fill the pot with about 1 to 2″ (2½-5cm) level of water & heat on a medium setting to start to boil. Immediately reduce the heat to low for it to simmer lightly & place the bowl on top of the warm pot.
Add the sugar, juice of 1 lemon & the pureed fruit to the bowl before mixing together. Allow the heat to melt down the sugar and become a liquid mixture. Move your pot & pan to a smaller ring/burner, intended for a small sized pot & reduce the heat to low-medium. Heating on a smaller ring/burner will ensure the eggs don’t get cooked too quickly. Alternatively, check your mixture isn't too hot, reduce the heat, or remove for a little bit if need be. Lifting up the bowl to let heat escape helps, but be very careful & protect your hands & arms first.
Whisk the eggs before adding in 2 or 3 increments to the mixture, as well as just 1tsp of the orange extract & mix through. Add in the cubed butter, one or two small cubes at a time, whisking in-between each addition, till the butter melts. Continue adding & whisking for about 15 minutes until thickened & reduced in volume.
Alternatively, for hands/arms that tire easily, use an immersion blender & whisk attachment or the dough hook(s) on a hand-held mixer. Whisk for no more than 10 seconds at a time, on lowest possible setting, before adding in the next cube of butter. Repeat until all butter pieces have been added.
Every so often check thickness of the curd with a wooden spoon, by coating the back of it and trying to draw a clear line through the mixture. For more on this see the detailed step-by-step process before this Recipe Card. Note it will thicken more off the heat & on chilling
Remove from the heat & from the cooking pot and allow to cool for 10 minutes before adding another 1½ tsp of orange extract. Add more to taste if you like.