Begin by prepping the rhubarb: chop the rhubarb into bite size pieces. I like to split each stalk at least in half lengthwise (if not thirds) so that you don't get any huge chunks. After slicing, chop into 1/2 to 1 inch pieces.

Add all the rhubarb to a bowl, along with 1/2 cup grenadine syrup. Stir it all together so that the rhubarb is coated. Let sit for 1-2 hours on the counter stirring occasionally, or even overnight (put it in the fridge, covered, if you go overnight.) This is to get the rhubarb a nice bright red color. It is not a necessary step but it sure is pretty!* When you are ready to make the rhubarb bars, drain the rhubarb in a colander.

While the rhubarb soaks, prepare the shortbread crust. Preheat the oven to 375 degrees F.

Line an 8x8 or 9x9 inch square pan with parchment paper, or spray with nonstick spray. Set aside.

In a large bowl or stand mixer, add 1 cup (2 sticks) softened butter. Beat until smooth and creamy, making sure to scrape the sides and bottom of the bowl.

Add 3/4 cup granulated sugar and beat again for at least 1-2 minutes, stopping to scrape the sides and bottom, until the mixture is light and fluffy.

Add 1 and 1/2 teaspoons vanilla extract and beat.

Add 2 cups of flour, spooned and leveled, and 3/4 teaspoon kosher salt.

Beat in the flour and salt until the mixture has come together and has formed a ball. Scrape sides as necessary. Do not over mix; beat just until the flour is combined and then no more (otherwise your shortbread will be tough).

Divide the dough into two, one with a little more than half the dough. (Cover the smaller piece of dough and put it in the fridge.)

Press a little more than half of the dough into the prepared square pan. Getting your hands a little damp can be helpful. I like to leave the edges raised a half inch or so, so that you have a bit of crust on the edge pieces, but pressing it all flat will work too. If you have time, chill the dough for a few minutes, 10-20 minutes is best. Cold dough going into a hot oven makes for very tender shortbread.

Bake the crust at 375 for about 15 minutes, or until light brown on the edges. My shortbread puffed up in the middle even though I had made a raised crust, so I gently pressed the center back down with the back of a spoon, when it was very hot from the oven still.

Set the crust aside and let cool for a few minutes.

Leave the oven on but lower the temperature to 350 degrees F.

Meanwhile, prepare the rhubarb filling: drain the rhubarb in a colander or strainer. Add your red rhubarb to a medium bowl.

Stir in 3/4 cup sugar (add a couple extra tablespoons if you skipped the grenadine soak*).

Add 1/4 cup all purpose flour and stir.

At this point you can add 1 tablespoon lemon juice and 1 teaspoon zest, but it is optional. It brightens up the flavor of the rhubarb, but you can definitely taste the lemon. Feel free to add just the juice and skip the zest for a milder hint of lemon. If you are a rhubarb purist, skip the lemon entirely.

Pour the rhubarb filling over the baked crust.

Use you fingers to break off medium pieces of shortbread from your stash in the fridge. Check the photos to see the size we're talking. I like big ol chunks. You don't have to cover the rhubarb entirely.

Bake the rhubarb bars in the oven at 350 for about 45-50 minutes. The bars are done when the shortbread on top is golden in several spots across the top. A fork should also slide easily into a piece of rhubarb; it should be completely tender.

Let the bars cool completely. I stuck mine in the freezer to speed this up.

If you would like to top with a vanilla glaze, in a medium bowl combine 1 cup powdered sugar with 2 teaspoons milk, 1/2 teaspoon vanilla, and a pinch of salt. Whisk until smooth. Drizzle over the top of the cooled bars and let the glaze set up for 20-30 minutes before slicing and serving.