Preheat oven to 375°F. Prepare the topping by mixing all the ingredients in a bowl until it turns into uniform clumps. Refrigerate until ready to sprinkle on muffins.
Prepare regular muffin tins with 7 LARGE liners. (You can bake regular sized muffins by dividing it into 12) I used mini copper soufflé moulds, but they are just as good made in regular muffin tins.
Whisk together the flour, baking soda, baking powder and salt in a bowl, set aside.
Cream the butter, sugars and zest together on medium speed, in a stand mixer with the paddle attachment, until light and fluffy, about 4 minutes. Add vanilla. Add the egg and yolk, one at a time, mixing on low just until combined. It may seem a bit curdled, because it is a lot of liquid to add to that amount of butter. It will all come together in the end.
Add half the flour and half the sour cream. Mix on low until combined. Add the remaining flour and sour cream.
Toss the blueberries with flour and if using frozen berries immediately fold them into the batter with two or three gentle stirs. If you over mix, the batter will turn purple and then gray. Scoop into the muffin liners.
Sprinkle the topping over the muffins. Bake for about 35-40 minutes or until golden brown and a tester comes out with moist, but not wet, crumbs. If you are baking smaller muffins, bake for 20-25 minutes. Cool slightly and serve warm or at room temperature.