- Preheat oven to 375°F.
- Prepare the streusel topping: Combine all the ingredients in a bowl and mix with your fingertips until it turns into small uniform clumps. Refrigerate until ready to sprinkle on muffins.
- Prepare the blueberry muffins: Whisk together the flour, baking powder, baking soda, and salt in a bowl, and set aside.
- In a stand mixer fitted with a paddle attachment, cream the butter, sugars, and lemon zest together on medium speed 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 mixture and half the sour cream. Mix on low until combined. Add the remaining flour and sour cream. Mix on low until just combined.
- Fold the blueberries into the batter with two or three gentle stirs.
- Prepare a muffin tin with 12 regular-sized muffin liners. Scoop batter into the muffin liners.
- Top each muffin with a few extra blueberries, then sprinkle the streusel topping over the muffins. Bake for 20 to 25 minutes or until golden brown and a tester comes out with moist, but not wet, crumbs. Cool slightly and serve warm or at room temperature.
Adapted from "Zoë Bakes Cakes: Everything You Need to Know to Make Your Favorite Layers, Bundts, Loaves, and More" by Zoë François © Ten Speed Press 2021. Provided courtesy of Zoë François. All rights reserved.