Mel the butter and add the sweetener, salt, cinnamon and vanilla. Mix well using a whisk in a mixing bowl and then add the eggs. Keep whisking and add lemon juice.
Mix all the dry ingredients in another bowl and start adding them to the wet mixture spoon by spoon. At this point, you can work the dough with your hands. Cover the bowl with plastic foil and leave it in the refrigerator for 10 minutes.
Form small balls from the dough (approximately 24). Place them on a baking sheet lined with parchment paper. On each cookie place 1 half of the walnut and tap it in.
Bake them at 180ºC (356ºF) for 15 minutes. Serve when cookies are cool.