In a large bowl, mix together melted butter, sugar, salt, and egg yolks until completely combined.

Add the flour and mix until just combined.  Be careful not to over mix.

Measure out two cups of the shortbread dough and press it into the bottom of a 9x13 inch baking pan.  The remaining crust mixture will be the topping, set it aside.

Slice and peel apples, making sure to slice them thinly for even cooking.  Use a good baking apple for this, a variety that holds its shape and won't turn into a sauce.  

Add the remaining apple filling ingredients (sugar and spices).  Toss to coat the apples completely.

Spread the apple slices over the shortbread crust.

Using your hands, crumble the remaining shortbread crust into pea-sized pieces and top the apples with the crust crumbles.

Bake the apple pie squares at 350 degrees for 50 to 60 minutes, until the top is browned and the apple pie filling is bubbling.

Allow the bars to cool before cutting and serving.