Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or grease each cup with non-stick cooking spray for easy removal.
In a spacious mixing bowl, combine the all-purpose flour, granulated sugar, baking powder, baking soda, and salt. Use a whisk to mix these dry ingredients until they are fully integrated.
In a separate bowl, whisk together the buttermilk, melted butter (make sure it's not hot), eggs, lemon zest, and vanilla extract until the mixture is smooth and well combined.
Carefully pour the wet ingredients into the bowl with the dry ingredients. Using a spatula or wooden spoon, gently fold them together. Mix only until combined, taking care not to overmix the batter; a few lumps are perfectly fine!
Gently fold in the fresh blueberries with a spatula, trying to avoid breaking them to keep the muffins juicy and vibrant.
Prepare the crumb topping by combining the flour, brown sugar, softened butter, and cinnamon in a small bowl. Use a fork or your fingers to mix these ingredients together until a crumbly texture forms.
Divide the muffin batter evenly among the muffin tin cups, filling each about two-thirds full. Generously sprinkle the crumb topping over the batter in each cup, ensuring a delightful crunch on top.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean, indicating they are perfectly baked.
Allow the muffins to cool in the pan for about 5 minutes before carefully transferring them to a wire rack to cool completely.
Notes
For a stunning presentation, arrange the muffins on a decorative platter and dust them lightly with powdered sugar.