Preheat the Oven: Start by preheating your oven to 180°C (160°C for fan-forced) or 350°F. Line a muffin pan with cupcake liners to make removal and serving easier.
Sift the Dry Ingredients: In a medium bowl, sift together the cake flour and baking powder. Next, add caster sugar and fine sea salt, then whisk everything until well combined.
Prepare the Wet Ingredients: In a separate mixing bowl, crack the eggs and whisk them together with the seeds from one vanilla bean until just combined. Then, add the buttermilk, melted butter, and neutral vegetable oil into the mixture. Stir it all together until it’s well incorporated.
Combine Wet and Dry Mixtures: Gradually fold the dry flour mixture into the bowl with the wet ingredients. Be careful not to over-mix; you want the batter to be smooth and well combined without being too vigorous.
Fill the Liners: Pour the batter into the prepared cupcake liners, filling each one about two-thirds full. This allows space for them to rise in the oven.
Bake the Cupcakes: Place the cupcake tray into the preheated oven. Bake for 16-18 minutes, or until a toothpick inserted into the center comes out with just a few crumbs attached. This indicates they are perfectly baked.
Cool the Cupcakes: Once baked, carefully remove the pan from the oven and let the cupcakes cool in the pan for a few minutes. After that, transfer them to a wire rack to cool completely.
Prepare the Frosting: For the vanilla bean frosting, whisk together the whole milk, half of the caster sugar, and the plain flour in a saucepan over low-medium heat. Cook this mixture until it forms a very thick paste. Once thickened, transfer it to a plate, spread it out, and cover with plastic wrap. Allow it to cool to room temperature.
Cream the Butter and Sugar: In a stand mixer, beat the softened butter with the remaining caster sugar until light and creamy. This should take a few minutes of mixing.
Incorporate the Vanilla: Scrape the seeds from another vanilla bean and add them to the butter mixture, along with a pinch of salt, and beat well to combine.
Combine Frosting Ingredients: Gradually add the cooled milk paste to the butter mixture, beating until the frosting is fluffy and smooth.
Decorate the Cupcakes: Finally, transfer the luscious vanilla bean frosting to a piping bag and pipe or spread it over the cooled cupcakes. Feel free to get creative with the decoration!