Preheat the oven to 350°F. Line a 12 cup muffin tin with liners, set aside.
In a large bowl, whisk together the self-rising flour, sugar, and salt, set aside.
In a medium-sized bowl, whisk together the butter, sour cream, egg, milk, and vanilla until smooth.
Add the wet ingredients to the dry and stir it in until combined and there are no dry patches, do not overmix.
Fold in the blackberries.
Evenly distribute the batter among the liners. Set aside.
For the topping, in a medium-sized bowl, whisk together the flour, brown sugar, and cinnamon.
Add the butter and cut it in with a pastry cutter, or the back of a fork until you break up the butter and the mixture resembles wet sand with a few larger chunks of butter about the size of peas mixed in.
Add the topping evenly on top of the muffins.
Bake for 18-22 minutes, until a toothpick inserted into the center comes out clean. A few crumbs sticking to the toothpick is okay but there should be no wet batter.
Let cool for two minutes in the tin then transfer the muffins to a wire rack to cool. Serve warm or at room temperature.