Preheat the oven to 350 and butter a 12-cup muffin tin.  In a medium bowl, whisk together the flour, baking powder, baking soda and salt, and in a small bowl, mash together the 3 topping ingredients (using your hands is easiest!) and set this aside until you’re ready for it.

Using your electric mixer, beat together the butter and sugar for a couple of minutes until fluffy, then add in the sour cream, egg, vanilla and zest and beat again until smooth.  Scrape down the sides of the bowl as needed.  Add the whisked dry mix to this wet mix and turn the mixer on low until the batter starts to come together.  Scrape down the bowl, mix again, then carefully fold in the blueberries by hand with a spatula.

Equally distribute the batter among all 12 muffin cups, then carefully top and press down a tablespoon or 2 of the cornflake topping onto each muffin – use all of the topping!

Bake for 20 to 25 minutes until the muffins are a lovely golden brown and the middles bounce back to a light touch. Allow the muffins to sit in the tin for about 10 minutes before removing each to a rack to cool.