Greek Yogurt Muffins with a swirl of Nutella in the middle.
The proper way to measure flour using measuring cups is to aerate it first. This is done either by sifting flour, or aerating it by fluffing it up and whisking it well, then spooning it into the measuring cup, then carefully removing any excess flour with a knife. If you just stick that measuring cup in the bag of flour and scoop some out, you will get a lot more flour than what the recipe calls for. Do aerate the flour, or you will end up with dry dough!
Combine brown sugar, flour, cinnamon, and butter in a small bowl. Mix until you have crumb-like mixture.
Use one regular size muffin pan (12 muffins). Line muffin cups with muffin liners.
Preheat oven to 400 degrees Fahrenheit.
In a medium bowl, mix together flour, white sugar, salt, and baking powder.
In a separate large bowl, add vegetable oil, 2 eggs, and Greek yogurt and whisk to combine.
Add flour mixture to the liquid mixture and mix just enough to combine.
Fill each muffin cup with the muffin batter about 1/3 full. Add 1 tablespoon of Nutella to each muffin cup. Add more muffin batter on top of Nutella to make muffin cups 3/4 full. Sprinkle the streusel topping over muffins. Bake for about 20 minutes in the oven.