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

Hazelnut Streusel

• 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

• 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.
• 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.
• 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.
• 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.
• 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!)
• Serve and enjoy!