1. Preheat the oven to 425°F / 220°C. Line a muffin tin with six parchment liners in an alternating order.

Hazelnut Streusel

1. In a small mixing bowl, melt the butter. Add in the sugar, flour and crushed hazelnuts, and mix until little clumps of dough have formed. Set aside.

Banana Muffins

1. In a medium sized mixing bowl, beat together the softened butter and sugar for 2-3 minutes or until the mixture is light and fluffy.
2. On a separate plate or in a small bowl, mash the banana. Then, add the banana, greek yogurt, and vanilla to the butter mixture. Beat until well combined.
3. Add in the flour, baking powder, baking soda, and salt. Lightly mix the batter together until just a few flour streaks remain. Pour in your add-ins of choice, then mix just until no flour streaks remain.
4. Distribute the batter evenly between the six parchment liners. If using, add the chocolate hazelnut spread to a piping bag fitted with a large round tip and pipe ~1 tbsp. of it into the center of each muffin. Then, top each muffin with streusel.
5. Bake the muffins for 5 minutes. Then, without opening the oven door, reduce the heat to 350°F / 175°C and continue baking for 10-15 more minutes, or until a toothpick inserted into the center of the muffins comes out clean. (The toothpick test is hard to use here if you piped chocolate into the center - in that case, carefully give the muffins a little push on the top - if they're firm, they're ready to be taken out!)
6. Serve and enjoy!