If you need to make quince puree, wash your quinces, remove the little black bit at the base and wash off the fluffy covering.Chop the quinces and put them in a heavy-based saucepan or preserving pan with enough water to cover. Bring to a simmer and cook until completely soft and pulpy. It will take about 25 minutes.
Push the pulp through a nylon sieve into a clean bowl. When you have sieved it all, measure it. You will need roughly 1.2kg of quince to make a kilo of quince puree.
Place the puree and sugar in a large preserving pan and simmer very gently for 1–1½ hours until it is really thick. Be careful as it can spit, you need it to be so thick that, when you scrape your spoon across the bottom of the pan, it leaves a clear channel before closing up again.
It also starts to come away from the sides of the pan as you stir, forming a thick mass.
Brush the silicone muffin pans with flavourless oil, this will help you de-mould the membrillo. Pour the mixture into the moulds.
If you want to give the membrillo as a present, or want your moulds back, you can unmould it and wrap in greaseproof paper or baking parchment, then tie with string. Properly wrapped, it keeps for a year in the cupboard or refrigerator. I find it stays moister in the refrigerator, but gets a bit sticky.