Add the cashews to a pot and cover with water. Boil for about 20 minutes over low heat to soften.
Preheat the oven to 350 degrees Fahrenheit.
While the cashews are boiling, we'll make the shortbread crust. First, add the butter to a mixing bowl and melt in the microwave. This usually takes about 40-45 seconds on high.
Once melted, add in all remaining crust ingredients, making sure to spoon and level the flours, don't scoop from the bag.
Mix with a wooden spoon to combine. The crust should be thick, but not dry. If it seems too wet, add an extra tablespoon or so of flour, too dry add a tablespoon of milk.
Add to an 8x8 pan lined with parchment paper. Press down with your fingers to spread evenly in the pan. It'll seem like there isn't enough crust, but I promise there is!
Bake for 15 minutes or until the edges are slightly golden brown.
Once the cashews are done boiling (they should look kind of puffed up and be soft), drain and rinse and add to a high speed blender.
Take your can of coconut milk and scoop the white part into a measuring cup. Add the remaining water until you reach 1 cup. It helps to smoosh down the white part to measure accurately.
Add the coconut milk along with all remaining ingredients to the blender.
Process until completely smooth, about 1 full minute.
Taste and adjust as desired. The sour from the lemon will fade slightly as the bars bake, but if it's too sour for you, either add more sugar or a splash of maple syrup. If it's too sweet, add more lemon juice.
Once the crust is done baking, add the filling to the crust and spread around until smooth.
Bake for 18-24 minutes, depending on your oven. After about 20 minutes, the center should still be kind of jiggly and not completely set, but also not raw.
Once done, let cool at room temperature for about 20 minutes, then transfer to the fridge for at least 4 hours, but ideally overnight to set. If you cut them prematurely, the filling won't hold it's shape well, so try to be patient!
Top with powdered sugar and lemon zest and slice into 16 bars. Enjoy!