Mix the flour, poppy seeds, lemon zest, baking powder, baking soda, and salt in a large mixing bowl.
In a medium bowl mix together the non-dairy milk, sugar, oil, lemon juice, and vanilla.
Add the wet ingredients to the dry ingredients and mix until combined, without over-mixing. Over-mixing can activate gluten and result in a tough cake instead of a soft cake.
Grease a 9 x 13 baking dish. Pour the batter into the baking dish and spread it out. Bake for 35 minutes, or until a toothpick comes out clean.
Let the cake cool completely. Slice and serve or add an optional lemon frosting.
For the frosting, cream together the butter with the powdered sugar. Add the lemon juice and whip. Spread onto the completely cooled cake.
Notes
Don't over-mix the cake. Over-mixing can cause the gluten to activate, leading to a tough cake.Thoroughly grease the cake tin for easy removal.Serve with the lemon frosting!