Lower oven rack to lowest position and preheat oven to 350 °F. Grease a 9”x5” loaf pan with non-stick spray and line with parchment.
Make Flax Eggs: Whisk together ground flaxseed meal and water in a small bowl and let sit for at least 5 minutes.
Make Buttermilk: Combine milk and ACV and let sit for at least 5 minutes.
Whisk together dry ingredients and set aside: flour, cinnamon, baking powder, cream of tartar, and salt.
Using an electric mixer with paddle attachment, cream the butter and sugar on Medium High speed until smooth and fluffy, about 2 minutes.
Reduce speed to Medium and slowly add the flax eggs, followed by the Buttermilk mixture and vanilla extract and mixed until combined.
Reduce speed to Low and gradually add the flour mixture until combined, scraping down the sides of the mixing bowl if necessary.
Pour the batter into prepared loaf pan and bake for 45-50 minutes, or until the top of the loaf is springy and a toothpick inserted into the center comes out clean.
Place Loaf pan on wire rack to cool for 30 minutes, then remove from pan. Allow loaf to cool for an additional 1-1.5 hours or until room temperature.
Cover and store in refrigerator for up to 1 week or slice and freeze.