Preheat oven to 350°F. Line an 8x8" brownie pan with parchment paper.

Make the crust. In a large bowl mix together the cassava flour, maple syrup, water, vanilla, lemon zest and cinnamon. When a dough starts to form, use your hands to knead the mixture into one ball of dough. If your dough is a bit dry, add a tablespoon of water to help shape the dough.

Press dough into the baking dish. Use a fork to poke a few holes into the crust. Bake for 40 minutes, until the shortbread is crisp. Let cool 10 minutes.

While the shortbread is baking, prepare the filling. In a saucepan, whisk the filling ingredients together. Heat over medium-low heat until the mixture thickens into a paste. It should no longer seem like a liquid. (I recommend starting this when then shortbread has about 10 minutes left in the oven.)

Spread the curd mixture onto the slightly cooled crust. Bake for 15 minutes, then remove from the oven and let cool at room temperature for 30 minutes. Transfer to the refrigerator for an additional 1-2 hours, or overnight, until set. (The curd will turn opaque.)

Lift the bars out of the pan and cut into squares. Serve with a lemon slice, if desired, and enjoy!