If using the crumble topping, prepare that first. Stir together the melted butter, sugar, and cinnamon, then add in the flour and stir until it resembles damp sand. Set the topping aside.

Preheat the oven to 375°F. Whisk together the dry ingredients (flour, baking powder, salt, cinnamon, and nutmeg) in a large bowl until well combined.

In a separate bowl, whisk together the yogurt, sugar, melted butter, oil, egg, and vanilla extract.

Pour the wet ingredients into the bowl of dry ingredients and stir until they're about 75% combined (there should still be pockets of dry flour throughout).

If using frozen blueberries, dust them with about 1/2 tsp flour first. Add the blueberries to the batter and finish folding until the blueberries are evenly incorporated and there are no more dry pockets of dry flour in the batter. It's okay if it's lumpy, just make sure to avoid over stirring.

Divide the batter between eight wells of a greased or lined muffin tin. Top the unbaked muffins with the crumble topping.

Bake the muffins for about 35 minutes, or until golden brown on top. Allow the muffins to cool for about 5 minutes in the muffin tin, then loosen the edges with a knife and transfer to a wire rack to finish cooling.