To make the jam, combine all the ingredients into a medium sauce pan.

Bring to a simmer and cook for about 10 minutes until the rhubarb breaks down and the jam is thickened.

Set aside the jam to cool at room temperature.

Combine the dry ingredients and set aside.

Preheat the oven to 350'.

In another bowl, beat the butter until light and fluffy.

Add the egg yolks and sugar and beat until the sugar is dissolved (you can't feel it gritty between your fingers) and the mixture is light.

Add the dry ingredients and continue to beat just until it is a rough crumb. This is where I believe a hand-held mixer is superior to the flat-beater type. I simply stop beating when my crumb is formed and forget the chilling and grating of the shortbread dough as per the original recipe.

Line your pan with parchment paper. Empty 1/2 the crumb mixture into the pan. Invert a cake decorating nail upside down in the center of the pan. This will help the center bake more evenly with the sides.

Gently press the crumb mixture into place.

Pour the rhubarb jam (or about 1 1/4 cup store-bought jam) over the shortbread base. Spread evenly with an off-set spatula.

Sprinkle the rest of the crumb topping onto the jam layer.

Bake the shortbread at 350' for about 40 - 50 minutes. The only way I know to check that the center is cooked is to take a small paring knife and poke around the middle. The jam is so wet that you can't tell if the shortbread is done without looking.

When done, the shortbread will be a darker golden brown.

When baked, sprinkle liberally with powdered sugar. Allow to cool at room temperature.

Let cool completely in the pan before cutting. Sprinkle with additional powdered sugar if it looks like the other has melted into the shortbread.

This freezes well as a block that can be cut later.