Make the shortbread crust:

1. Preheat. Position a rack in the middle of the oven and preheat the oven to 350°F (180°C). Butter an 8-by-8 inch square baking pan and line two sheets of parchment paper crosswise, extending up the four sides, forming a “sling” for your bars. The parchment paper makes it easy for you to remove the bars and slice them later.
2. Mix using a stand mixer: In the bowl of a stand mixer fitted with the paddle attachment, mix together the flour, sugar, and salt on medium-low speed to combine.  Add the butter pieces, one at a time, and mix until the butter is incorporated and has a wet, crumbly appearance, about 4 minutes. Continue mixing until the dough comes together and forms a ball, about 3 more minutes. Mix by hand: In a large bowl, whisk together the flour, granulated sugar, and salt. Add the butter pieces, one at a time, and using your hands or a pastry blender, break up the butter in the mixture until a wet, crumbly mixture forms, about 8 minutes. Mix, using a wooden spoon, until the dough comes together and forms a ball, about 3 minutes.
3. Bake. Transfer the dough to the prepared baking pan and press it evenly across the bottom. Bake for 20-25 minutes, until the edges are slightly golden. Remove the pan from the oven and let it cool to room temperature on a baking rack. 

Make the Frangipane:

1. Combine the wet ingredients. In the bowl of a stand mixer fitted with the paddle attachment, or a large bowl with a hand mixer, beat the butter and sugar on high speed until fluffy and no butter pieces are visible, about 1 minute. Add the egg, and mix on medium speed until just combined. 
2. Add the dry ingredients. Remove the bowl from the mixer (if using) and stir in the almond flour and salt with a spatula.

Assemble the bars:

1. Top the crust. Spread the frangipane over the cooled crust in an even layer.
2. Arrange the rhubarb. The easiest way to tackle any pattern is to cut the rhubarb as you arrange the rhubarb on the frangipane. For a basketweave pattern, start by cutting a few pieces of rhubarb into 2-inch batons and arranging the pieces in the middle of the baking pan. If you have small pieces of rhubarb leftover, set them aside; they are perfect to fill the gaps in the pattern later. Continue cutting the rhubarb and moving outwards toward the edges of the pan. Fill in any gaps with the small, leftover pieces at the end. 
3. Bake the bars. Bake until the frangipane is golden brown on top and the rhubarb is tender, about 50 minutes. 
4. Serve. Remove the baking pan from the oven and transfer it to a wire rack. Let the bars cool completely in the baking pan. For the cleanest cuts, refrigerate the bars until they are very cold before cutting. Cut into 16 2×2″ squares or your desired size.