Preheat the oven to 180°C conventional or 160°C fan-forced. Line a 12-hole muffin tin with cupcake cases.
Sift the flour, baking powder, bicarbonate of soda and salt into a bowl.
In a separate large bowl, beat the softened butter and caster sugar for 2–3 minutes until pale and fluffy.
Add the vegetable oil and beat until combined. Add the eggs, one at a time, beating briefly after each addition.
Mix in the vanilla bean paste and vanilla extract.
Whisk the buttermilk and sour cream together in a small bowl until smooth.
Add half the flour mixture to the butter mixture and mix gently. Add the buttermilk mixture, followed by the remaining flour. Mix only until the batter is smooth and no dry flour remains.
Divide the batter between the cupcake cases, filling each approximately two-thirds full.
Bake for 18–20 minutes, checking from 17 minutes. The cupcakes are ready when the tops spring back gently and a skewer inserted into the centre comes out with a few moist crumbs.
Leave in the tin for 5 minutes, then transfer to a wire rack to cool completely.
To make the buttercream, beat the softened butter for 4–5 minutes until very pale, smooth and fluffy.
Add the sifted icing sugar gradually, beating on low speed between additions.
Add the vanilla bean paste, vanilla extract, salt and 2 tablespoons of cream. Beat for another 2–3 minutes until light and silky. Add the remaining cream if needed to reach a soft, pipeable consistency.
Pipe or spread generous swirls of buttercream over the cooled cupcakes and finish with plenty of sprinkles.
