01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or grease each cup lightly.
02 - In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until evenly distributed.
03 - In a large bowl, whisk the melted butter with the sugar until smooth. Add eggs one at a time, whisking well after each addition, then stir in the vanilla extract.
04 - Add the dry ingredients to the wet mixture in three additions, alternating with the milk in two additions, mixing gently after each. Stop as soon as no dry streaks remain to avoid overmixing.
05 - Gently fold the raspberries into the batter using a rubber spatula, taking care not to crush the berries.
06 - In a small bowl, combine the flour, sugar, and cinnamon. Add the cold cubed butter and rub between your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
07 - Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full. Sprinkle the streusel topping evenly over each muffin.
08 - Bake for 20 to 22 minutes, or until a toothpick inserted into the center of a muffin comes out clean and the tops are golden.
09 - Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before serving.