Pre-heat the oven to 170 C (165 C fan) and line the 12-hole cupcake/ muffin tin with liners/ cupcake cases.
Place a medium sieve over a bowl of your electric mixer and sift flour, sugar and baking soda into the bowl.
Add eggs and butter and mix on low speed until it just comes together, then increase the speed to medium and mix until you have smooth batter, about 1 minute. Scrape the sides and the bottom of your bowl as needed. Add milk, vanilla extract and vanilla bean paste (if using) and mix again on medium-high for about 30 seconds. Scrape the sides and the bottom of the bowl again to make sure all the ingredients are incorporated together. You should have a smooth, slightly thick batter.
Divide the batter equally between 12 cases. Do not overfill your cases, you want to fill up max of ¾ of each case. You may want to use a measuring scoop here to ensure equal amount of batter goes into each cupcake.
Place the cupcakes into the middle shelf of the oven and bake for 22 minutes, until toothpick inserted in the middle comes out clean and they are golden brown on top. Remove from the oven and allow them to cool for 5 minutes, before removing from the baking tin into a cooling rack.
In a bowl of an electric mixer (or freestanding mixer with the paddle attachment), beat together the butter and icing sugar on low until smooth, light and fluffy, about 2 minutes. Add the vanilla extract and salt and beat again for 30 seconds. If the consistency is too stiff, add 1 Tbsp of milk to loosen it up.
Transfer the buttercream into a piping bag fitted with the nozzle of your choice (I used Wilton 2D) and pipe it on top of your cupcakes.
To make your own self-rising flour, add 2 tsp of baking powder for each 150g / 1 cup of plain flour and mix well together.
To achieve that smooth, even and gently risen cupcake top and to avoid 'humps', it's crucial your butter is at the room temperature. Test the butter by poking it with your finger. It should leave an indent without sinking or sliding down into the butter.
Do not overmix your ingredients. Stop mixing as soon as they are combined.
This recipe makes 12 cupcakes or 24 mini-cupcakes. If making mini-cupcakes, reduce the baking time to about 12 minutes (or as soon as the toothpick inserted in the middle comes out clean)