Preheat your oven to 350°F. Grease or line a 9x9 or 8x8 baking pan with parchment paper.
Add the 3 eggs, ½ cup mayonnaise, ⅓ cup cottage cheese, and 1 teaspoon vanilla extract to a blender. Blend just long enough to get rid of the curds but not so much that it turns to liquid.
Pour the blended mixture into a large bowl. Add the 1⅔ cups oat flour, ½ cup cocoa powder, 1 cup powdered sugar, 1½ tablespoon arrowroot powder, 1 teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon salt, and ¼ cup hemp seeds. Stir until just combined.
Pour in the ¾ cup brewed coffee and mix well. The batter will become lighter and slightly bubbly - that's exactly right!
Pour the batter into your prepared pan and bake for 20–25 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs. Do not overbake.Let the cake cool completely in the pan before frosting. While the cake cools, make the frosting. Add the 1¼ cups cottage cheese, ⅔ cup hazelnut spread, ½ teaspoon salt, 2 teaspoon vanilla extract, and 1 tablespoon of the milk to a blender. Blend until completely smooth and creamy, scraping down the sides as needed, and add the additional tablespoon of milk as needed. All curds should be gone.
Spread the frosting evenly over the cooled cake. Top with flaky sea salt just before serving, slice into 12 pieces, and enjoy.