Sieve 100g sugar and 300g flour together, combine with 200g cold butter cut into pieces, and one egg yolk.

Knead together until one rather sticky glob of dough. (It will at first seem as if all those ingredients never could stick together - and eventually becomes so it seems as if more flour might be necessary.)

Let rest in the fridge for a while. (30 minutes)

Take out the ball of dough, knead it to warm it a bit (cold, it would just break apart and/or be too solid to work), flour the baking board/surface and the dough well, and roll the dough out to about 5 mm thickness. It will be necessary to take good care the surface underneath the dough and the surface of the dough (or the roller) are well-dusted with flour, or the dough will just stick and crumble apart. Even so, some tendency for the dough to break must be expected…

Cut out circles, with a cookie shape if you have it, but a stronger glass of the desired width does the trick very well, too. Put the cut-out cookies onto parchment/baking paper on baking trays. Knead together remaining dough and cut out further cookies, repeat until dough is used up.

Bake the cookies for about 15 minutes at 180C; they should get golden-brown but not burnt-brown - let the thinner cookies be your guide. When done, take out of the oven and let cool well. (Do not try to hurry things and lift the cookies off when they are still warm or they'll just break and crumble!

Dissolve 20g of vanilla pudding mix in a few tablespoons from 80g of milk, combine with one egg yolk. Heat the rest of the milk with 20g sugar dissolved in it. When boiling, take off heat and slowly whisk together the two liquids. Heat a bit, whisking constantly, until creamy. (Take care, it does not take long at all, wants to all settle into thick clumps if there's too much heat or too little whisking.) Melt 120g of baking chocolate (the one here is typically 40% cacao, so I don't add further sugar; darker one would be better but should get sugar added) and ~60g of butter. Whisk together with the pudding crème.

For Devil-Ischler: Add chile pepper to the cream filling. I recommend a teaspoon of chile ancho for just a bit of its sweetness, aroma, and spiciness. But, if you feel more devilish, go ahead and use a bit of habanero powder!

For Japan-Ischler: Use white instead of dark chocolate and add 1-2 teaspoons of matcha.

Leave to cool out and become firm(er).

Spread a good tablespoonful of the cream filling on one cookie, softly press another on top to get the crème to the edges but not over. If professionalist, spread crème as needed to get it evenly distributed. If the cream got a bit too firm, just stir it to make it more creamy before using it.

Note: A simpler version of Ischler would only see them filled with apricot or other jam/marmelade; for an even more luxurious version, first spread a thin layer of - I'd suggest, sour-sweet - jam on the bottom cookie, or also on the top, and put the cream in the middle.

Melt ~150 g chocolate and 60-100g butter (or e.g. coconut oil) slowly and stir until completely combined.

Glaze the filled/combined cookies with the melted chocolate. (More chocolate in the melt will make a crisper layer but be much harder to spread; more fat makes the glazing easier to spread; the professionalist may want to spread the glaze around the side with a spatula or butter knife, but that's too much hassle even for me.)

I'm saying that a bit late, but: Putting the set-together cookies onto a wire rack, with the previously-used parchment paper underneath to catch chocolate dripping down, is a good idea for that.

If so inclined, decorate with a walnut half after glazing. Those also make for a good axis, one finger at the bottom of the cookie, one on the walnut, around which to nibble the cookie into oblivion ;)

Put in a cool place or the fridge again and let cool out and solidify; these are no cookies you can eat immediately (unless you want to have a chocolatey-crumbly mess)!