Walnut Cookies (Oreshki)

Beat the eggs with the sugar and salt until well blended. Stir in the melted butter. In a small dish, stir the baking soda with the vinegar. It will start to fizz; pour this into the the egg mixture and beat well. Stir in flour until well blended.

Depending on which type of mold you have, grease the mold with vegetable oil. If your oreshki maker says not to grease, then follow manufacturer instructions.

If you are using the shells that go in the oven, pre-heat oven to 350F and line the shells with a thin, 1/8-inch thick lining of dough, trimming off the edges. Bake 8 to 10 minutes or until golden. Remove from shells as soon as you can handle them. (see notes for video**)

If you are using an oreshki iron (either an electric one, or one that is heated over a gas stove), follow manufacturer directions, or if you don't have them, form teaspoons of dough into small balls. Pre-heat an electric iron, then place one ball in each cavity; press and hold until golden. If your iron is the kind you use over a gas stove, turn it over mid-way. An electric iron will not need to be turned, of course. (see notes for video**) Set aside to cool.

With a sharp knife, gently clean up edges of baked oreshki so that they are smooth and uniform. Reserve the crumbs for later; don't throw them away. At this point, you can freeze your oreshki for later use, or store at room temperature in an airtight container.

Before serving (or up to 1 week in advance of serving) fill each shell with dulce de leche and sandwich half-walnuts together to form whole walnuts. The easiest way to do this is to put the dulce de leche in a piping bag and pipe it into the shells, but you can also fill them with a spoon. If desired, stuff the center with a walnut piece. Put the reserved crumbs into a zip-loc bag and pound with a rolling pin until crumbs are small and uniform; transfer to a small bowl. Roll filled oreshki in crumbs so that any filling oozing out between the two halves is covered in crumbs. Store filled oreshki in an airtight container up to 1 week in the refrigerator. If desired, dust with powdered sugar before serving.