Prep: reheat oven to 400 degrees F. Line a muffin pan with silicone baking cups or paper liners and set aside.
Combine the dry ingredients: In a small bowl, whisk the flour, kosher salt, and baking soda for the muffins. Set aside.
Combine the wet ingredients: In a large bowl using either a stand mixer or a hand mixer, beat the salted butter and granulated sugar for 3 to 4 minutes. Add the eggs 1 at a time, beating after each addition. Add the sour cream and vanilla extract and beat for 1 minute.
Combine the dry and wet ingredients: Slowly add the dry ingredients to the butter sugar mixture. Mix until just combined. Set aside.
Make the streusel: In a medium sized bowl, combine the streusel ingredients. Stir until well combined and crumbly pieces are formed. Set aside.
Fill the pan: Divide the muffin batter evenly in the prepared muffin pan using a large cookie scoop. Top each muffin with a generous amount of the streusel and press down slightly. If desired, top muffins with 4 to 5 blueberries.
Bake the muffins: Bake for 17-20 minutes or until a toothpick comes out clean when inserted into the middle of the muffins.
Cool and dust the muffins: Remove the muffins from the oven. Cool 5 minutes. Dust with powdered sugar.