1. Preheat your oven to 400°F and either grease a muffin tin or prepare it with muffin liners.

Make the Crumble

1. Melt the unsalted butter (3 Tablespoons) and set aside to cool for about two minutes.
2. In the bowl of a stand mixer fitted with the paddle attachment, add the all-purpose flour (1/2 cup, see note*), granulated sugar (1/3 cup), and kosher salt (1/8 teaspoon). Mix together on low for about thirty seconds.
3. With the mixer still on low, slowly pour in the melted unsalted butter (3 Tablespoons) and allow to continue mixing until a crumbly mixture forms.
4. Set the crumbles aside in their own bowl as the mixing bowl and paddle will be used to make the muffin batter. There’s no need to wash the paddle and the bowl before making the batter.
   

Make the Muffin Batter

1. In the bowl of a stand mixer fitted with the paddle attachment, add the unsalted butter (8 Tablespoons, softened to room temperature),  granulated sugar (1/2 cup), light brown sugar (1/4 cup – packed), vanilla extract (2 teaspoons), kosher salt (1/2 teaspoon), and baking powder (2 teaspoons).

   Mix on medium speed (level 4 if using a KitchenAid stand mixer) for 5 minutes until lightened and fluffy, scraping down the bowl and paddle with a rubber spatula as needed.
   

2. With the mixer still on medium speed, add the eggs (2, large) one at a time, allowing to incorporate entirely between additions.
   
3. Pause the mixer and add the sour cream (2/3 cup) to the bowl, using this as a chance to again scrape down the bowl and paddle with a rubber spatula. Return the mixer to medium speed and allow to combine. It will not become smooth, but instead have a slightly grainy-looking texture.
   
4. With the mixer still on low, slowly drizzle in the vegetable oil (1/4 cup) – it should take about a minute to add all of the oil, then mix until combined.
   
5. Remove the bowl from the mixer and add the all-purpose flour (2 cups, *see note) to the bowl. Gently fold together by hand with a rubber spatula until almost entirely combined.
6. Add the fresh or frozen blueberries (2 cups) to the mixing bowl and gently fold everything together by hand until the blueberries are evenly dispersed in the batter and there are no dry flour patches. Be careful not to burst any of the berries to avoid the batter turning purple.

   If using frozen blueberries, toss them in a Tablespoon of cornstarch before adding them to the batter so that they don’t sink to the bottom. The batter will also firm up a bit due to the cold temperature, but they’ll still turn out great.
   
   

Assemble and Bake the Muffins

1. Fill the prepared muffin tins with scoops of muffin batter so that each tin is 2/3 full.

   I find a large cookie scoop to be the easiest way to portion batter evenly into the muffin tins.

2. Evenly top each muffin with the prepared crumble until no crumble remains.
3. Bake for 18-20 minutes or until a toothpick or knife can be inserted into a muffin and comes out clean.

   If using frozen berries, the muffins will take a few extra minutes to bake through. Be sure to test if they’re baked by inserting a toothpick or knife and baking until it comes out clean.

4. Let cool in the pan before removing and enjoying.