Preheat your oven to 180C (356F) or 160C (320F) for a fan forced oven. Line a cupcake tin with cupcake liners.
In the bowl of a stand mixer fitted with the paddle attachment, add the flour, baking powder, caster sugar and salt. Turn mixer on low speed and allow it to mix for a couple minutes to help everything combine well (alternatively you may do this by sifting the ingredients together). Add the softened butter and let it mix until it resembles a fine sand like texture.
Next, add milk, eggs, yogurt, oil and vanilla extract in a large jug and whisk well.
Add wet ingredients to dry ingredients in a slow and steady stream until no dry ingredients are visible. Scrape down the bowl and mix for another 20 seconds. Add 1 cup coloured sprinkles and gently fold in about 4 times. Any more than that and the colors will run through the batter causing it to turn grey.
Fill each cupcake liner 3/4 of the way (using an ice cream scoop to transfer the batter into the cupcake liners makes this a quick and easy process otherwise you can use two tablespoons). Bake for 20-25 min or until a toothpick inserted comes out clean. Allow them to cool completely on a wire cooling rack before frosting.
Fit the end of your piping bag with a Wilton 1M piping tip and frost in a swirl motion as demonstrated. Finish off with a drizzle of chocolate sauce and a sprinkle of coloured sprinkles.
Store in an airtight container for up to 3 days (refrigerated or unrefrigerated).