Preheat oven to 350°F.
Arrange hazelnuts on a sheet pan and toast until dark golden brown 15-20 minutes, shaking the pan every 5 minutes so they don’t burn. Remove from the oven and let cool slightly.
Weigh 57 grams of the toasted hazelnuts into the bowl of a mini food processor. Process until the hazelnuts are a very fine powder, almost like a nut flour. Transfer to a large mixing bowl and set aside.
Rough chop the remaining hazelnuts and set aside (these will get mixed in later). This can be a fairly large chop — you don’t want any whole hazelnuts, but halves and quarters are fine.
To the ground hazelnuts in the mixing bowl, add all purpose flour, sugar, salt, baking powder and whisk to combine.
In another bowl, whisk together eggs and buttermilk. Then pour in the melted butter, whisking constantly. Whisk vigorously until very well combined. The butter will firm up if the egg and buttermilk are too cold so make sure they’re truly lukewarm before you add the butter!
Use a spatula to gently fold the wet ingredients into the dry ingredients. You don’t want to develop gluten here, so the goal is to mix in as few movements as possible. When there is just a bit of dry mixture still visible, add the chopped hazelnuts and mini chocolate chips. Continue folding just until no dry bits remain and the inclusions are evenly distributed. Do not overmix.
Cover and let sit 30 minutes at room temperature.
Line a standard muffin tin with paper liners. Use a large cookie scoop to divide the batter between the tins. You may have extra batter — divide it between the tins or into any that need a bit more.
Top each muffin with ½ teaspoon granulated sugar (mandatory) and quick rolled oats (optional).
Bake 20-22 minutes, until a toothpick inserted into the center comes out clean.
Let cool 5-10 minutes in the pan before transferring to a cooling rack to finish cooling completely.