Preheat the oven to 350ºF, adjust a rack to the middle position. Spray a 12 cup regular size muffin tin with some nonstick spray.
Whisk flour, salt, and baking powder in a large mixing bowl and set aside. Whisk egg, sugar, and vanilla together until combined, then whisk in the sour cream and melted butter.
Scatter half the berries in the flour mix and scatter the rest in the sour cream mixture. Add the sour cream mixture to the flour and fold to combine, take care not to smush the raspberries too much. There should be not large pockets of flour but small streaks may remain. Do not over mix. The batter will be very thick.
Divide the batter evenly into the tin (a very large cookie scoop is excellent for this). Mix all of the ingredients for the streusel together and top each muffin with a small spoonful, then spread the filling out over the top of the muffin.
Bake for about 20 – 25 minutes or until a toothpick inserted in the center comes out clean. Gently pick the muffins out of the tin and cool them on a rack, flipping them out may cause the topping to fall off.