Position a rack in the middle of the oven and preheat to 350º F. Grease a large muffin tin with melted butter.
Whisk the dry ingredients (hazelnut flour, salt, baking soda, and cinnamon) together in a large bowl. In a separate bowl, whisk together the wet ingredients (eggs, oil, honey, vanilla, sweet potato, and dates). Pour the wet ingredients into the dry and whisk to combine. TIP: measure the oil and honey with the same measuring cup, measuring the oil first. The thin coating of oil that remains in the measuring cup helps release the honey without sticking.
Portion the batter evenly into the muffin tin. Bake for 20 minutes, or until a toothpick inserted into the middle of the muffins comes out clean.
While the muffins bake, prepare the frosting. Using an electric hand mixer or a whisk, combine the butter, cream cheese, and vanilla until fluffy. Add the honey and beat until smooth and similar in texture to cake frosting. Taste and add more honey if the frosting isn't sweet enough for you.
Let the muffins cool for 10 minutes in the tin, then carefully remove and let cool completely.
Transfer the frosting to a piping bag or ziploc bag with the tip of one corner cut off. Frost each muffin in a swirl. Chill to solidify and serve cold.