1. Preheat the Oven:

1. - Set oven to 375°F (190°C).
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2. - Line a 12-cup muffin tin with paper liners or lightly grease with non-stick spray.
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2. Prepare the Blueberries:

1. - In a small bowl, toss the blueberries with 1 tablespoon of flour to coat. This helps prevent them from sinking during baking. Set aside.
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3. Mix Dry Ingredients:

1. - In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon (if using). Set aside.
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4. Mix Wet Ingredients:

1. In a separate medium bowl, whisk the sugar and eggs until light and frothy, about 1 minute.

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2. Add the vegetable oil (or melted butter), milk, sour cream (or yogurt), vanilla extract, and lemon zest (if using). Whisk until smooth and fully combined.

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5. Combine Wet and Dry Ingredients:

1. Pour the wet ingredients into the dry ingredients. Gently fold together with a rubber spatula or wooden spoon until just combined. Avoid overmixing; a few small lumps are okay.

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2. Gently fold in the flour-coated blueberries, being careful not to crush them.

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6. Make the Streusel Topping (Optional):

1. In a small bowl, combine the sugar, flour, and cinnamon. Add the cold cubed butter.

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2. Use your fingers or a fork to mix until the mixture resembles coarse crumbs.

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3. Sprinkle evenly over the muffin batter after filling the tin.

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7. Fill the Muffin Tin:

1. - Divide the batter evenly among the 12 muffin cups, filling each about ¾ full. A large cookie scoop works well for this.
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2. If using streusel, sprinkle it generously over the top of each muffin.

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8. Bake:

1. Bake for 18–22 minutes, or until a toothpick inserted into the center of a muffin comes out clean or with a few moist crumbs.

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2. If using frozen blueberries, baking may take 1–2 minutes longer.

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9. Cool:

1. Allow muffins to cool in the tin for 5 minutes, then transfer to a cooling rack to cool completely or serve warm.

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10. Serve and Store:

1. Enjoy muffins warm or at room temperature.

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2. Store leftovers in an airtight container at room temperature for up to 2 days, or refrigerate for up to 5 days. Muffins can also be frozen for up to 3 months; thaw at room temperature or reheat in a 300°F (150°C) oven for 5–10 minutes.

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