Preheat the oven to 425°F and prepare a 12-cup muffin tin with paper muffin liners.
Add flour, baking powder, sugar, and salt in a bowl and give it a whisk with a fork.
In a mixing bowl combine the eggs, vanilla, melted butter, sour cream, oil, and mashed bananas and whisk together.
Add the liquid ingredients to the dry ingredients.
Gently fold just until the dry ingredients are wet. Lumps are okay when making muffins. You do not want to mix until the batter is smooth.
Gently fold in the blueberries, just until lightly coated in batter.
Line a cupcake tin with paper liners. Scoop 1/4 cup into each cupcake liner.
In a small bowl, combine streusel ingredients and then sprinkle 1-2 tablespoons over each cupcake batter.
In a preheated 425-degree oven, bake for 12 minutes until golden brown.
Notes
Don't over-mix them or your muffins will be dense and heavy.Fill the muffin cups with batter until they are two-thirds full, and bake them until lightly golden brown.Use room temperature ingredients when making muffins.Baking times will vary, so check the muffins after 10 minutes and see what they need. These delicious blueberry banana muffins can last up to 3 days in an air-tight container. This will help retain their freshness and promote longer storage times.Alternatively, muffins can be kept in the fridge for up to 5 days or wrapped and frozen for up to 4 weeks.To reheat any muffins stored in the fridge or freezer, pop them into a preheated oven for a few minutes until warm, and enjoy.
A good way to tell when your muffins are done baking is to check if the top is lightly browned and a toothpick inserted into the center comes out clean.You should also gently press down on the top of one muffin with your finger; if it springs back up, your muffins are ready to take out of the oven.