Preheat & Prep: Preheat oven to 400°F. Line a muffin pan with paper liners.
Mix Dry Ingredients: In a small bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt.
Combine Wet Ingredients: In a large bowl, mix eggs, sour cream, milk, both sugars, melted butter, and vanilla. Use an electric mixer. The batter will be VERY thick.
Combine & Fold in Blackberries: Gently mix the dry ingredients into the wet ingredients with a few light strokes—don’t overmix.
Lightly coat the berries in a small amount of flour before folding them into the batter. This helps prevent them from sinking. The batter should be lumpy, not smooth.
Fill Muffin Cups: Divide the batter evenly among the muffin tins.
Make Streusel Topping: In a bowl, mix all streusel ingredients until crumbly. Evenly distribute the topping over each muffin and press down gently.
Bake: Bake for 15-18 minutes, or until a toothpick inserted in the center comes out clean. Could take a few more minutes depending on ovens
Cool & Serve: Let muffins cool in the pan for 2-3 minutes, then transfer to a wire rack. Enjoy warm or at room temperature.