Preheat the oven to 170C/325F/Gas 3. Line a 12-cup muffin tin with muffin cases, and set aside.
Whisk together the flour, baking powder, baking soda, and salt. Set aside.
Whisk together the eggs. Add the sugar, and whisk until combined. Add the vanilla extract, butter, and yogurt, and whisk until combined. Fold in the dry ingredients. Gently fold in the sprinkles.
Divide the batter evenly between the muffin cases, filling only 2/3 full.
Bake for 15 - 20 minutes, or until a toothpick inserted into the centre comes out clean. Allow to cool for 5 minutes in the pan, before transferring them to a wire rack to cool completely before frosting.
For the Frosting
Using a handheld or stand mixer, beat the butter until pale and creamy - about 5 minutes. Add half of the icing sugar, and beat for 3 minutes, then add the remaining sugar and beat for another 3 minutes until combined.
Add the milk, and vanilla, and beat until smooth and fluffy.
Transfer the frosting to a piping bag, and frost the cooled cupcakes. Decorate with sprinkles!
Notes
*Make your own cake flour! Measure out 1 and 1/2 cups (190g) of plain/all-purpose flour, remove 3 tablespoons, and replace with 3 tablespoons of cornflour/cornstarch. Sift well.Leftover cupcakes can be stored, covered tightly, in the fridge for up to 3 days. Frosted or unfrosted cupcakes can be frozen for up to 3 months. Thaw overnight in the fridge.Cupcakes can be made 1 day in advance, covered, and stored at room temperature. Cover the frosting, and store in the fridge until ready to use.