2. Grease an 11x7-inch baking glass dish with butter. Line the dish with a parchment paper sling, leaving an overhang of about 2 inches on each long side.
3. Combine the flour, cane sugar, powdered sugar, zest, and salt in the bowl of a food processor fitted with the blade attachment and pulse to combine.
4. Add the butter and pulse until it looks like coarse crumbs.
5. Press the crust into the greased pan and chill in the refrigerator for 15 minutes.
6. Par bake the crust until light golden-brown, 25 to 30 minutes.
7. In a separate bowl, whisk the whole eggs, egg yolk, cane sugar, powdered sugar, zest, salt, lemon juice, cassava flour, and lemon essential oil.
8. Remove the parbaked crust from the oven and pour the lemon filling onto the warm crust.
9. Return the pan to the oven and bake until light brown around the edges, about 15 to 20 minutes.
10. Cool completely on a wire rack before cutting.
11. Remove the lemon bars from the pan using the parchment sling.
12. Use a fine-mesh strainer to generously dust the bars with powdered sugar.
13. Store your grain-free lemon bars in a sealed container in the fridge; they will last up to one week.