Preheat the oven to 350°F (175°C) and line a cake pan with parchment paper.
In a large mixing bowl, whisk together the grated carrots, eggs, and milk until well combined. Add the oat flour, baking powder, and cinnamon to the bowl and whisk until the batter is smooth. Pour the batter into the prepared cake pan and smooth it out with a spatula.
Bake the cake for 25-30 minutes in the preheated oven, or until a toothpick inserted into the center comes out clean. Once the cake is finished baking, let it cool completely in the pan.
Make the frosting. In a small bowl, whisk together the Greek yogurt and honey until well combined.
Once the cake has cooled, remove it from the pan and spread the frosting over the top and sprinkle with chopped walnuts or sliced almonds. Slice the cake and serve.