Preheat the oven to 220 °C (or 200 °C fan-bake) and grease a 12 muffin tin.
Make the streusel by combining all the streusel ingredients together. Stir it with a fork or rub it with your fingers to make a rough crumble. Set it aside.
To make the muffins, add the butter, granulated sugar and brown sugar into a mixing bowl.
Use an electric mixer to cream the butter and sugar together until light and fluffy.
Beat in the eggs, one at a time, followed by the vanilla, cinnamon and salt.
Add the vinegar to the milk.
Add the flour, baking soda and baking powder to the butter and egg mixture and fold it together, along with the milk + vinegar. Fold it gently until it's all combined.
Stir through the blackberries, breaking them up a little bit as you do so.
Divide the batter into the muffin tin. Sprinkle the streusel overtop, pressing it lightly into the batter to help it stick.
Place the tray in the oven and lower the heat to 200 °C (or 180 °C fan-bake.) Bake the muffins for around 18-22 minutes until a skewer inserted comes out clean.
Let the muffins cool for 5 minutes before removing from the tray.