Cream the softened butter with the egg and sugar until smooth.

Slake the baking soda with vinegar or lemon juice, add it to the butter mixture and mix again.

Sift in one cup of flour and stir it in. Then gradually add the remaining flour and knead a smooth dough.

Pinch off pieces of dough about the size of half a walnut and roll them into balls. For a classic stovetop mold make them bigger - the size of a whole walnut; electric makers have smaller cavities.

Grease the mold before the first batch, place the dough balls in the cavities and bake the shells until done.

Remove the baked shells, let them cool, then trim the excess edges with a knife to neaten the shape.

Fill the shells with the boiled condensed milk and press them together in pairs. If you like, tuck a piece of walnut into one of the halves.

That's it - help yourself and enjoy!