Using a handheld or stand mixer fitted with a whisk or paddle attachment beat the butter with the sugar on high speed for about 3 minutes.
On medium speed, add the eggs one at a time, beating well after each egg is added.
Beat in the sour cream, milk, and vanilla extract.
On low speed, add the dry ingredients with the wet ingredients in one bowl together. Beat until all dry ingredients are well incorporated.
Toss the blueberries in flour for a good coating.
Gently fold the blueberries into the muffin batter. * please read important note below
Spoon the batter into muffin liners, filling them ALL the way to the top. ** please read notes below
Top each with the streusel mixture.
On 425 degrees F, bake the muffins for 5 minutes.
Reduce the oven temperature to 350 degrees F and bake for 20 minutes, check with a toothpick. If it comes out clean, then remove from the oven. If moist batter is on the toothpick, bake for another 5 minutes. I prefer to bake for 25 minutes total, just in case the bottoms of the muffins aren't fully baked due to the blueberries.
Allow the muffins to cool and rest IN the muffin pan for 5 - 10 minutes.
Remove them and place on a wire rack to continue to cool.