Preheat oven to 375 degrees. Line a muffin tin with 10 paper liners or spray each cup with a nonstick spray.

Beat butter and sugar with an electric mixer until light and fluffy.

Add egg and beat well, then sour cream and zest.

Whisk together the flour, baking powder, baking soda and salt into a bowl. A little at a time, add the dry ingredients into the wet mixture. Mix until combined.

Gently fold (by hand) in your blueberries. The dough will be thick, and it will resemble cookie dough. Using an ice cream scoop or spoon, fill your muffin cups about 3/4 of the way full.

To make the streusel topping: Combine the sugar, flour, and cinnamon in a small bowl. Cut in the COLD butter till the mixture resembles crumbs. I tend to use my hand to cut in the butter but if you have a pastry blender you can use that. Or two knives work well too.

Sprinkle a generous amount over the muffin batter, pressing slightly to adhere it to the batter. (It can get a bit messy on the pan..but no worries)

Bake for 25 to 30 minutes, until tops are golden and a tester inserted into the center of muffins comes out clean (you know, except for blueberry goo). Let cool on rack - if they make it to cooling.