First, prepare the lemon curd. In a small pot, whisk together the nondairy milk, water, and cornstarch.
Place over medium-high heat and add in the lemon juice, lemon zest, sugar, and vegan butter, continuing to whisk. Mixture should start bubbling and thickening. Whisk for 1-2 minutes as it boils, then remove from heat and mix in the lemon extract. Set lemon curd aside to cool until needed. It has to be COLD before it can be used in the cake.
Next, make the cake. Preheat oven to 350F and spray two 6" round cake pans with nonstick baking spray. In a small bowl, whisk together the ground flaxseed with the water and set aside to thicken.
In the bowl of a stand mixer, combine the flour, baking powder, baking soda, salt, and sugar.
Add in the vegan butter, and beat on low until it's incorporated and has a sandy texture.
In a small bowl, whisk together the oil, flaxseed mixture, nondairy yogurt, lemon zest and and lemon extract, and add them to the dry ingredients. Mix until combined.
Finally, pour in the unsweetened nondairy milk and lemon juice, and beat on medium-low until smooth and creamy.
Divide batter evenly among the 2 pans, and bake for 45 minutes until cake is set in the center and a toothpick comes out clean.
When lemon curd and cakes are completely cooled, you can make the frosting. In the bowl of a stand mixer, beat the shortening, vegan butter, and lemon juice together until completely smooth and creamy. Add in the powdered sugar, 1 cup at a time, until a thick frosting forms. You may need more or less sugar to get the right consistency. Add in the salt and some food coloring if desired.
To assemble the cake, divide the 2 cake layers in half to create 4 thin layers. Place one cake piece on your cake stand or platter, and pipe a circle of frosting around the edge. Gently spoon some lemon curd in the center, and spread it to create a thin layer. Repeat with all. Keep in mind you will have some lemon curd left over! Frost the outside of the cake as desired and decorate with ganache if you want. Top with fresh flowers.