To make the tart shells:
Preheat the oven to 400 degrees F. In a large bowl, whisk together the flour, sugar and salt. Stir in the butter, then the egg yolk, and then the almond extract. Use your hands to shape the dough. Pinch off a piece, roll it into a ball about the diameter of a quarter, and press it into the bottom and halfway up the sides of a mini muffin cup. You want the dough to be thin, as it puffs when it cooks. Repeat with the remaining dough.
Bake for 8 to 10 minutes, until golden. Let cool for 2 to 3 minutes only before carefully removing the shells from the pans (see Sugar Mommas Notes). This will prevent them from crisping too much and breaking as you remove them. If this does happen, however, just put the pans back in the oven for a couple of minutes to soften the shells a bit. Let cool completely before filling. The shells may be stored in a tightly covered container in the freezer for up to 1 month.
To make the filling:
Fill the bottom of a double boiler (see Sugar Mommas Notes) with 1 to 2 inches of water and bring to a rolling boil. Place the whole eggs and yolks in the top of the double boiler off the heat. Beat gently with a fork or whisk until the whites and yolks are thoroughly mixed. Place the top of the double boiler back in place over medium-low heat. Stir constantly, watching the eggs carefully so that they don't start to curdle (see Sugar Mommas Notes).
Add the butter, sugar, lemon juice and zest. Cook over gently boiling water, stirring often with a wooden spoon, until the mixture is the consistency of mayonnaise. This takes 10 minutes or so, and you do need to let it sit for a minute or two without stirring or it won't thicken. You can see it thicken around the rim of the double boiler. Remove from the heat and let set for 5 minutes. Pour the filling into a glass bowl and place a piece of plastic wrap across the surface (so a skin doesn't form) and refrigerate for at least 1 hour or up to overnight. Fill the tart shells with the filling and serve. The filling will keep in a covered container in the refrigerator if you dare (Momma Reiner would eat it within an hour) for up to 2 weeks. The shells can be frozen for up to 1 month. Leftover assembled tartlets will keep, covered, in the refrigerator for a few days.
To make the Sugar Mommas Meringue Topping:
Preheat the oven to 400 degrees F. In the bowl of a stand mixer fitted with the whisk attachment, beat the egg whites on high speed until foamy, about 30 seconds. Reduce the speed to medium, slowly add the sugar and vanilla, and mix until the whites are glossy, about 1 minute. Place 36 lemon tartlets (or less, if you do not wish to top them all) on a baking sheet and spoon about 1 heaping teaspoon of meringue over the top of each. Bake for 4 to 5 minutes, until slightly browned.
Sugar Mommas Notes:
You may use a pointed knife edge to help loosen the tart shells from their molds, then turn the pan over on parchment paper and use a knife to gently tap the bottom. The mini crusts should pop right out. If you don't have a double boiler, you may use a metal bowl nestled in a pot of boiling water (the water should remain at least 2 inches below the bottom of the bowl).
If your eggs scramble a little in the double boiler, do not fret. Use a spatula to push the filling through a fine-mesh strainer before serving to remove any little bits of egg so that it has a smooth consistency.
Modern variation: Use the leftover egg whites to make a meringue topping, or use whipped topping. Put the meringue or whipped topping on some, but not all, of the tartlets. That way you'll create a little diversity in your display.
Sass it up: If you're using meringue or whipped topping, place a single red raspberry on top to add color.