Preheat the oven to 425°F (218 °C) with a shelf ready just above the middle. Line 12 wells of a muffin pan with muffin liners.
To a large bowl add the flour, sugar, baking powder and salt. Whisk them together to combine.
Make a well in the centre of the flour mixture, then add the milk, oil and vanilla.
Mix together gently until just combined, then add the blueberries and fold them through. The batter is supposed to be thick.
Divide the mixture evenly between the muffin wells, filling them just shy of the top.
If using the optional blueberry jam, drop about a teaspoonful on the top of each muffin and swirl it in with the tip of a sharp knife.
Top each muffin with a few more blueberries and a generous sprinkle of sugar, or some vegan streusel, then place in the oven.
Immediately set a timer for 5 minutes. After that time is up turn the oven down to 375 °F (190 °C) (don't open the oven door) and cook for approximately 18 minutes, or until they are golden, well risen, and a toothpick or sharp knife inserted comes out clean, except for maybe a little blueberry juice. If you are using a dark coloured muffin pan they might be ready a minute or two earlier.
Leave in the pan for 10 minutes before transferring to a wire rack to cool completely.
STORING MUFFINS - Allow the muffins to cool completely on a wire rack then transfer to an airtight container with some paper towels underneath and on top. The paper towels help absorb any moisture. Muffins can also be frozen for up to 3 months.