In a bowl, whisk together eggs, sugar, oil and vanilla. Sieve and mix together the 4 cups plain flour, salt, baking soda and baking powder. Whisk half of the dry mixture into the eggs, followed by half of the Tatua Sour Cream. Repeat with the remaining dry ingredients and Tatua Sour Cream.
Fold the blueberries through the batter, then scoop evenly into a 12-hole lined muffin tray.
To make the streusel topping, mix together the remaining flour, brown sugar and cinnamon. Rub in the cold butter until a crumbly texture forms, then sprinkle evenly over the muffin mixture.
Bake in the oven for 20-25 minutes or until the streusel topping is golden and a skewer comes out clean when muffin is tested. Leave to cool slightly and serve or store in an airtight container for up to 3 days.