Line bottoms of two round 9-inch layer cake pans with parchment paper. Preheat oven to 350°F.
Whisk the flours, psyllium husk powder, soda, and salt together in a mixing bowl.
In another bowl, beat the eggs until they become foamy and pale. Beat in the sugar, oil, lemon juice, and vanilla.
Mix the dry mixture into the wet mixture, and pour it evenly into the two prepared cake pans.
Bake in preheated oven for 20 minutes or until a thermometer inserted into the centre reads 210°F (99°C). Be careful not to overcook it.
Allow cakes to cool on wire racks for 10 minutes. Then insert a dull knife or other thin, flat utensil between the cake and sides of the pan. Run it around the outside edges to separate the sides of the cake from the pan. Once you have loosened the edges, tilt the cakes upsides down onto the wire racks, peel off the parchment paper from the bottoms, and allow them to cool completely.
If you've not already done so, prepare a recipe of Lemon Curd. Chill it in the refrigerator.
Prepare a recipe of Cream Cheese Frosting and stir one teaspoon of grated lemon zest into it.
Once the cake layers have cooled completely, place one layer upside down onto a serving plate. (The rounded top side is down, and the flat bottom is up.) Cover this flat bottom generously with Lemon Curd.
Place the second cake layer, right side up, onto the lemon curd, so the flat sides are together with lemon curd between them.
Spread frosting over the cake. Begin with covering the sides first. Then spread frosting over the top. Sprinkle a second teaspoon of grated lemon zest over the top of the frosted cake for a decorative effect.
Store unused cake, covered, in the refrigerator.
Be careful not to overcook your cake, or it could dry out.
If you want to be fancy, use a piping bag and tip to frost the cake.
Use freshly squeezed lemon juice. The stuff in the bottle just isn't the same.