Preheat the oven to 200C/180C Fan/Gas 6. Line a 6 hole muffin tin.
In a large bowl, combine the dry ingredients (flour, baking powder, sugar).
Make a well, and pour in the liquid ingredients (milk, butter, oil and egg). Stir gently with a wooden spoon until just combined.
Add the cornstarch to the blueberries and toss so that all the blueberries are coated (I do this in the blueberry punnet to reduce washing up). Pour into the batter and stir gently.
Scoop 1 tbsp batter evenly into each muffin hole.
Use a teaspoon to scoop 1 tsp blueberry preserve in the centre of each muffin. Top off with enough batter to cover the preserve without overfilling the muffin cases (never go too far above 2/3rds full)
Make the streusel topping by combining all ingredients in a small bowl and sprinkle over the muffins.
Bake for 18-20 minutes, until golden on top. Remove and let cool on a wire rack - highly recommend eating whilst still warm!
