Preheat the oven to 400°F / 205°C. Line a muffin pan with 6 parchment liners.
Place the chopped hazelnuts in a sauté pan over medium heat. Stir occasionally until they become darker and are toasted. Set aside.
In a medium sized bowl, mash the banana.
In a small bowl in the microwave (or on the stovetop), melt the butter. Add the butter and sugar to the banana and whisk until well combined.
Whisk in the egg and vanilla until very well combined, then whisk in the sour cream.
Add in the flour, ground cardamom, baking powder, baking soda, and salt. Gently mix the mixture together just until all of the flour is almost combined. Add in the hazelnuts and gently stir just until no flour streaks remain.
Evenly distribute the muffin batter between the six parchment liners. Top each muffin with 1 tsp. turbinado sugar, if desired.
Bake the muffins at 400°F for 5 minutes. Without opening the oven door, reduce the temperature to 350°F / 175°C. Bake for an additional 10 minutes, or until a toothpick inserted into the center of the muffins comes out clean.
Serve and enjoy!