I got this recipe for the filling years ago from a sample issue of the magazine Cuisine at Home. It's a pretty standard recipe, though, and it's meant to be used in a large tart. One recipe of this is going to make enough for at least 72 tarts, so I have cut the filling amounts to fit with my standard crust recipe. Happily, the recipe was very easy to divide by three, and it's obviously easy to multiply from the small amount I list. As I've said with the other miniature tarts recipes, though, it's not really worth making these unless you make a lot. I usually put a strawberry slice on top of each, although a blueberry or two also works well. These are so pretty for some kind of spring event, such as a tea or an Easter dinner. It's necessary to pre-bake the crusts, by the way, for this particular recipe, because the filled tarts are baked so briefly and there wouldn't be a chance for the crust to brown.
Whisk the first five ingredients together in a medium heavy-bottomed saucepan. then add the butter, cubed. Cook over medium-low heat, stirring constantly, until filling thickens but is still pourable. The best tool for stirring this is a heatproof spatula or a flat whisk, which allows you to get into the very edges of the bottom and therefore prevent you from having scrambled eggs there. Cook to 160 degrees, checking periodically with your instant-read thermometer. Spoon or scoop a small amount into each pre-baked tartlet shell and bake at 325 degrees for about 5 minutes. Let cool and then chill.
Add toppings if desired: strawberry slices, blueberries, a raspberry--and brush some melted jelly over the top, the classic choice being currant jelly. But you can use whatever you have on hand. If it's jam, I'd advise straining it to remove seeds and pulp before you brush it on; otherwise, you'll have this blobby stuff on top of the fruit. Currant or apple jelly won't require straining, and it's easy to keep on hand for just such uses; you don't use very much. You can also put a little pouf of whipped cream on these or just sift powdered sugar over them..
*I make a point of saying that I think bottled citrus juices are perfectly acceptable and that I use them all the time. I always have lemon and lime juice in the fridge, and I also have citrus oils on hand to use in the place of zest. These oils come in little tiny bottles and are only available online; I buy the Boyajian brand through Amazon. They may seem expensive but in reality you use so little at a time that you won't need to replenish your supply for quite awhile.