Preheat the oven to 350 ºF (180 ºC). Lightly grease and flour a 9-inch pan.
Prepare the chia egg by adding 6 tablespoons of warm water to 2 tablespoons of chia seeds. Set aside for 10 minutes.
In a medium bowl, prepare the vegan buttermilk by adding one tablespoon of lemon juice to 5 tablespoons of plant milk (preferable soy). Set aside for 10 minutes.
Peel, core, and cube the apples. Toss the apple cubes with 2 tablespoons of lemon juice, the lemon zest, and the cinnamon powder. Set aside.
In a large bowl, sieve the dry ingredients. Whisk together the flour, brown sugar, and baking powder. Set aside.
Mix wet ingredients. To the buttermilk vegan bowl, add the chia eggs, vanilla, oil, and whisk until well combined.
Pour the wet ingredients into the dry mix and combine. Don't over mix (especially if using non-gluten-free flour). Once most of the flour is incorporated, fold in the apples.
Spread the batter into the prepared pan, sprinkle with extra brown sugar, and bake for 30-35 minutes until a toothpick inserted in the center comes out clean.
Enjoy at room temperature or cool.