Preheat the Oven:
Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or grease the cups.
Prepare the Dry Ingredients:
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt
Mix the Wet Ingredients:
In another bowl, whisk together the melted butter, buttermilk, eggs, vanilla extract, lemon zest, and lemon juice until well combined.
Combine Wet and Dry Ingredients:
Pour the wet ingredients into the bowl with the dry ingredients. Gently fold the ingredients together until just combined. Do not overmix; it's okay if there are a few lumps.
Gently fold in the blueberries until evenly distributed throughout the batter.
Fill the Muffin Cups:
– Divide the batter evenly among the muffin cups, filling each about 2/3 full. If using, sprinkle the streusel topping over the batter.
Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center comes out clean, and the tops are golden brown.