Preheat a fan-forced oven to 160C (320F) or 180C (356F) for a conventional oven. Line a cupcake tin with cupcake liners.
Add the flour, sugar, cocoa powder (optional), baking powder, baking soda and salt into a large mixing bowl and mix on low speed to help everything combine. You may sift it if mixing by hand.
Add the eggs, oil, buttermilk, vanilla and blue food dye into a large jug and whisk until well combined.
Slowly add the buttermilk mixture into the dry ingredients while mixing on low speed. Once al the wet ingredients are in there stop the mixer, scrape down the bowl, and mix for a final 20 seconds.
Scoop into cupcake pan lined with cupcake papers, filling up about ½ way and bake for 30 min. Cupcakes will look shiny and moist on top. If it bounces and doesn't sink when you touch it, it's done! Set them aside on a cooling rack to cool completely before frosting.
Once cooled, core the centre of each cupcake and fill with star sprinkles