Preheat & Prep: Set your oven to 400°F. Line a standard muffin or cupcake pan with paper liners and set aside.

Make the Crumb Topping: In a small bowl, combine the flour, sugar, and cardamom. Pour in the melted butter and mix with a fork until the mixture becomes crumbly. Place in the refrigerator until needed.

Prepare the Dry Ingredients: In a large mixing bowl, whisk together the flour, baking powder, and salt. Set aside.

Mix the Wet Ingredients: In a medium bowl, combine sugar with lemon zest and rub them together for about a minute to release the citrus oils. Add the eggs and whisk until well blended. Stir in the sour cream, 1/3 cup melted butter, lemon juice, and vanilla extract until the mixture is smooth and pale yellow.

Combine Wet & Dry Ingredients: Gently fold the wet mixture into the dry ingredients, stirring just until combined. Avoid overmixing.

Set aside 1/2 cup of blueberries for topping. Place the remaining blueberries in a small bowl, coat them with 1 tablespoon of flour, and gently fold them into the batter.

Divide the batter evenly among the prepared muffin liners, filling each about two-thirds full. Lightly tap the pan on the counter to help the batter settle.

Sprinkle the reserved blueberries over the batter, then generously top each muffin with the crumb topping.

Place the muffins in the oven and bake at 400°F for 5 minutes. Then, reduce the temperature to 375°F and continue baking for 13-15 minutes, or until a toothpick inserted into the center comes out clean.

Let the muffins sit in the pan for about 5 minutes before transferring them to a wire rack to cool completely