Preheat oven to 350 F.
Cream together butter and sugar.
Add egg yolk mash, flour, lemon juice, walnuts and cinnamon (enough to give the dough a nice golden brown color).
Chill the dough in the refrigerator for at least 1 hour. (overnight is good, too).
Divide the dough in half. (half for the base and sides, half for the lattice top).
Roll out the dough to between ⅛" - ¼" thickness.
Place in the baking pan to cover the bottom and sides. (it doesn't have to be one big piece - several pieces work fine).
Make holes in the dough with a fork.
Bake for 7 minutes.
Mix the jam in a bowl and add a little water so it's more spreadable. (I add a little lemon juice as well).
Gently spread the jam on the pre-baked dough.
Roll out the rest of the dough and cut into strips for the lattice.
Place strips in a pattern on top of the jam. Don't worry if the strips break - several pieces look just fine.
Bake for 25-30 minutes (jam should be bubbling). The sides often get a bit burnt - so watch out for that.
Use any leftover dough to make little cookies!