Oven at 400.
Make the topping by using a pastry cutter and mixing the first 3 ingredients (cold butter, brown sugar, flour) with a pastry cutter. DO NOT KEEP AT ROOM TEMPERATURE. Put it in the fridge until you are ready to top your muffins.
In a large mixing bowl, mix your dry ingredients (flour, white sugar, baking powder and salt.)
Add the brown sugar.
In a separate bowl, whisk your wet ingredients (melted butter, eggs, buttermilk and vanilla).
As your pour the wet ingredients into the dry ingredients, gently stir with the wooden spoon your mom never spanked you with. (I'm just checking to see who reads the recipes.)
Take your blueberries and toss with the remainder 1 Tbsp of flour. (This is how you prevent sunken blueberry syndrome....the flour absorbs some of the juice as it bakes and gives it a little more stability.)
Add the blueberries to the mixture...don't overmix.
Prepare your muffin tin with those cute little muffin holders
Divide your batter into each cup.
Add a few blueberries to the tops and sprinkle with the streussel in your fridge. Press it down a little so it doesn't come flying off when you grab it.
Bake for about 20 minutes.
Let cool in pan for a few minutes and then remove from pan.
Don't count them and divide them...eat them and share willingly.