Place the flour into a mixing bowl. Add the butter, xylitol, and baking powder.
Separate the eggs. Add the egg yolks to the flour mixture. Put the egg whites into a separate bowl for whipping later.
Work the dough by hand. Rub the butter into the flour mixture with your fingertips, then knead until smooth. Add enough milk to create a soft dough. Shape it into a ball.
Cut the dough ball in half to make it easier to work with. Roll it out on a floured surface to about 0.5 cm thickness. The thinner the dough, the easier it will fit neatly into the molds.
Take a glass or round cutter that matches the size needed for your tartlet molds. For my 6 cm tartlet molds, I used a 7.5 cm diameter glass.
Press each cut-out dough circle evenly into the molds, but not too firmly, otherwise they will be harder to remove later.
Once all the molds are filled, place one teaspoon of jam into the center of each pastry.
Prepare the walnut filling. Beat the egg whites into stiff peaks, then set aside.
Place the ground walnuts into a mixing bowl. Add the xylitol, erythritol, lemon zest, and rum. Mix thoroughly, then gently fold in the whipped egg whites.
Transfer the filling into a piping bag or a plastic bag with the corner cut off. Fill the tartlets with the mixture. Do not overfill them, because the meringue will rise to about double its size during baking.
You can also decorate the tops with sugar-free sour cherries or walnut pieces.
Bake in a preheated oven at 180°C (top and bottom heat with fan) for 20-25 minutes.
Once baked, remove the tartlet molds while they are still warm, because they come off more easily at this stage.