Preheat your oven to 350F and line an 8"x8" baking dish with parchment paper (not to be confused with wax paper!). In a medium mixing bowl, stir together the ground flax seeds and water, and allow them to gel together for about 10 minutes. If you haven't pureed your beets at this point, now is a great time to do so!
Add the pureed beets, buckwheat flour, honey, cocoa powder, salt and vanilla to the bowl with the gelled flax seed mixture, and stir well until a thick, uniform batter is created. Fold in the dark chocolate chips, if using.
Transfer the batter to the parchment-lined baking dish, and use a spatula to spread the batter evenly to the edges. Bake at 350F for 20-25 minutes, or until the center is fairly firm to the touch.
Allow to cool completely before serving. These brownies should last for a few days when stored at room temperature, but for best shelf life, store them in a sealed container in the fridge.
Notes
I attempted this recipe with both roasted beets and steamed beets, and I found that the flavor of the steamed beets was less noticeable in the final product, when compared to the flavor of the roasted beets. To steam the beets, I like to cut them into chunks (no need to peel) and place them in a saucepan, fitted with a steam basket, filled with 2 inches of water. Bring the water to a boil, then cover and allow to steam for 15 minutes, or until the beets are fork tender.