Preheat the oven to 375°F (190°C). Grease a standard 12-cup muffin tin or line it with paper liners.
In a medium-sized bowl, beat together the softened butter and granulated sugar.
Add the eggs one at a time, scraping the sides and bottom of the bowl and beating well after each addition.
Beat in the baking powder, salt, and vanilla.
Add the flour alternately with the milk, beating gently just to combine. Scrape the bottom and sides of the bowl.
Mash ½ cup of the blueberries. Add the mashed and whole berries to the batter, stirring just to combine.
Scoop the batter into the prepared muffin pan.
Make the streusel topping: In a small bowl, whisk together ¼ cup of granulated sugar, 2 tbsp of flour, and ½ tsp of ground cinnamon. Cut in the cold butter until the mixture resembles coarse crumbs.
Sprinkle about 1 tsp granulated sugar atop each muffin.
Bake the muffins for 30 to 35 minutes, until they’re light golden brown on top.
