Instructions

1. Preheat oven to 350 degrees F.
2.  Spray an 8x8 baking pan with cooking spray and line with parchment paper. (Crisscross the parchment in the pan to make flaps so you can easily lift out the cake.) Spray the top of the parchment lightly too. 
3. In a small bowl stir together cornstarch and water. Let sit 2-3 minutes.
4. In a medium bowl, add the sugar, melted butter, agave, lemon zest, lemon juice, and vanilla extract. Whisk well then add the corn starch mixture and whisk again.
5. To the liquid mixture, stir in the almond flour, coconut flour (make sure you packed your tablespoons), salt, and baking soda. Stir until batter forms. Fold in poppy seeds. Allow to sit and absorb liquids for 2-3 minutes.
6. Transfer batter to prepared pan.
7. Flatten the batter into an even layer, making sure all corners are filled. I did this with a pair of food safe gloves; Tessa suggests another piece of parchment.
8. Bake for 16-19 minutes. The edges will be slightly browned and the top golden brown. Remove from oven and cool completely on wire rack. 
9. While bars are cooling make the frosting: combine powdered sugar, vegan butter, lemon zest, vanilla extra and lemon extract (if using). Beat with an electric mixer until smooth. Add additional powdered sugar if needed. Store in refrigerator until ready to frost.
10. When cake is cool, refrigerate for at least 30 minutes.
11. Gently frost the cake. Grate additional lemon zest on top. Remove cake from pan using parchment flaps. Slice into squares or bars.
12. Store leftovers in an airtight container.