In a large microwave-safe bowl, melt the butter and set aside to cool.
Combine flour, salt, baking powder, and sugar in a medium bowl and set aside.
Add the eggs and vanilla into the bowl of the melted butter and whisk gently to combine, then add in the milk.
Add the dry ingredients to the bowl with the wet ingredients and mix until just combined (do not over mix). Fold in the blueberries.
Cover the bowl of batter and chill in the fridge for 30 minutes. This step is optional but highly recommended as it will help create much taller, beautifully rounded muffin tops *SEE NOTES
After the batter has been chilling for 20 minutes, preheat the oven to 400°F and line a muffin tin with 12 liners (or lightly spray the tin with cooking spray)
Make the streusel topping by combining brown sugar, granulated sugar, flour, and cinnamon in a small bowl, then mash in the butter with a fork until combined. (If you're not using streusel, skip this step).
Fill the muffin liners ⅔ full with the chilled batter, then generously sprinkle streusel topping over them. Bake for 13-17 minutes or until a toothpick inserted in the middle of a muffin comes out clean.
Let muffins cool in the tin for 5 minutes then transfer to a wire rack to cool completely.