I use the following recipe courtesy of Punk Domestics, it is always thick and delicious....not to mention simple:
Preparation: 15 – 20 minutes
Ingredients (yield: one 300-350 ml jar):
juice from 3 lemons
zest from 1 lemon
12 – 15 tablespoons castor sugar (or 10 tablespoons sugar + 5 tablespoons cooking sweetener which is usually sweeter than sugar)
2 eggs
1 flat tablespoon cornstarch (or potato starch, but cornstarch gives a lighter result)
1 heaped tablespoon butter
Mix everything in a blender, apart from the butter.
Pour into a small pan, add the butter and warm at low heat, constantly stirring, until it thickens.
Taste and add more sugar if needed. Stir well until the sugar/the sweetener dissolves.
Put into a jar, close the lid and let it cool down.
Keep in the fridge for up to two weeks.