Blueberry-Sour Cream Muffins

I've made these 3 times, and today I doubled the lemon zest and they are perfect! Tender, but not chewy Also, the first time I over filled the muffin cups so I only had 11, but they puffed up beautifully. 1 star off because the first time I followed the instructions exactly, using "softened butter" to make the topping, and not only was it tough to work with and portion like that, but it made the topping clump up on the muffins. The next time 2 times, I used very cold butter which I grated into the flour/sugar/cinnamon mix and they came out perfectly. I used approx. 2 Tbsp butter and half the listed amounts of flour/brown sugur and that was more than enough to generously cover 12 muffins. Update: I've been making these at least once a month. The first few times I made them with vanilla extract, but the last 2 times I used lemon extract. Both were good- maybe it was the brand of lemon extract I used, but I didn't taste any extra lemon flavor. If you're planning to freeze them, don't make the topping. It doesn't hold up as the muffins thaw.