In a small bowl, beat the egg white with a hand mixer on high speed until soft peaks form. Set aside.

In a medium bowl, add remaining donut ingredients and use a hand mixer on medium speed, for 30 seconds to blend together. Fold in egg white and stir gently to combine.

Place the mixture in a piping bag or ziploc bag with the corner cut off and fill the cavities of a 6-donut mold tray 3/4 full. Gently tap the tray on the countertop to settle the mixture. Place in the oven and cook for 18-20 minutes.

While the donuts are cooking, make the blackberry glaze. Add the blackberries and lemon juice to a food processor and pulse until pureed. If you'd like the blackberry glaze de-seeded, strain the puree through a fine mesh sieve into a small pot. Use the back of a large spoon or spatula to press the puree through the sieve. If not de-seeding the puree, transfer the puree to a small pot.

Heat the blackberry puree on low heat and add the coconut butter and maple syrup. Add one tablespoon of water at a time to reach the desired consistency and turn off the heat.

Once the donuts have finished cooking, remove from the pan and let cool for a minute or two. Dip the donuts halfway in the glaze to coat. Sprinkle lemon zest on top.